The invention relates to a drinking beaker according to the preamble of claim 1. Drinking beakers of this type are known from US 6098834. With the known drinking beaker, the valve is permanently closed through the resilient means unless the user operates a lever. The disadvantage of the known drinking beaker is that when the drinking beaker is being cleaned, for example in a dishwasher, the valve remains closed, as a result of which dirt will remain behind between the valve and the lid. It is an object of the closable drinking beaker according to the invention to overcome this drawback and to this end, it is designed according to the characterizing clause of claim 1. If the lid is cleaned separately from the beaker, the valve is detached from the lid without movement means having to be operated, and complete cleaning can be effected. In accordance with one embodiment, the closable drinking beaker is designed according to claim 2. In this case, the valve and thus the drinking nozzle are closed at the same time the lid is closed. In accordance with one embodiment, the closable drinking beaker is designed according to claim 3. As the pin is free on all sides when the lid is detached, the hinge coupling can be cleaned in a simple manner. In accordance with one embodiment, the closable drinking beaker is designed according to claim 4. The user sucks air through the air supply opening and thus drinks liquid mixed with air, so that hot liquid is cooled during drinking by means of the air which is sucked along. In accordance with one embodiment, the closable drinking beaker is designed according to claim 5. This results in a simple construction for operating the valve. In accordance with one embodiment, the closable drinking beaker is designed according to claim 6. Consequently, the hinge point between the beaker and the operating arm is at some distance from the lid and the valve, as a result of which small movements of the operating arm on the side of the beaker enable relatively large movements of the valve, thus facilitating operation. In accordance with one embodiment, the closable drinking beaker is designed according to claim 7. This results in the operating arm being protected against undesirable movements when the beaker falls on the ground and prevents opening of the valve and leaking from the beaker, for example when the latter falls on the ground. In accordance with one embodiment, the closable drinking beaker is designed according to claim 8. In this way, the support is reinforced in a simple manner. In accordance with one embodiment, the closable drinking beaker is designed according to claim 9. Consequently, the lid can be opened by being turned while the operating arm is placed in a protected position between ribs attached to the beaker. In accordance with one embodiment, the closable drinking beaker is designed according to claim 10. Consequently, the movement means are strong while at the same time being easy to operate. In accordance with one embodiment, the closable drinking beaker is designed according to claim 11. As a result, the operating means can easily be operated by the hand which holds the closable drinking beaker. In accordance with one embodiment, the closable drinking beaker is designed according to claim 12. As a result, the pneumatic control can also be used with great differences in temperature. The invention will be described below with reference to a number of exemplary embodiments using a drawing, in which: Figure 1 shows a diagrammatic section of a first exemplary embodiment of a closable drinking beaker, Figure 2 shows a plan view of the first exemplary embodiment shown in figure 1, Figure 3 shows a section III-III of the first exemplary embodiment shown in figure 1, Figure 4 shows a diagrammatic section of a second exemplary embodiment of a closable drinking beaker, Figure 5 shows a part of the side view of the second exemplary embodiment shown in figure 4, Figure 6 shows a plan view of the second exemplary embodiment shown in figure 4, Figure 7 shows a section VII-VII of the second exemplary embodiment shown in figure 4, Figures 8a-8d show various views of a third exemplary embodiment of a closable drinking beaker, Figure 9 shows a section of a lid of a fourth exemplary embodiment of a closable drinking beaker, and Figure 10 shows a perspective view of an alternative embodiment of a lever with a valve as can be used with the exemplary embodiments of figures 8 or 9. In these embodiments, identical parts are denoted as much as possible by the same reference numerals. Figures 1, 2 and 3 show a closable drinking beaker 1. The closable drinking beaker 1 consists of a more or less cone-shaped beaker 2 and a lid 4. The lid 4 can be screwed onto the beaker 2 by means of a screw thread 3, a sealing ring 5 being disposed between the beaker 2 and the lid 4, in such a manner that no liquid can leak from between the beaker 2 and the lid 4. The screw thread 3 may be designed as a single-thread screw thread. If desired, other known types of screw threads may be used, such as a triple-thread screw thread, optionally with interrupted threads, as a result of which the lid 4 can quickly be fitted onto the beaker 2. It is likewise possible to use a bayonet fastening or also a sliding seal. The lid 4 is provided with a drinking nozzle 7 with a drinking aperture 8. The drinking nozzle 7 is in communication with the interior of the beaker 2, the end of the drinking nozzle 7 facing the beaker 2 can be closed by means of a valve 6. The valve 6 is designed in such a manner that it is normally closed and the liquid present in the beaker 2 cannot flow out of the beaker 2. The valve 6 is provided with operating means so that it can be opened by someone wishing to drink from the drinking nozzle 7. During opening, the valve 6 moves in the direction of the beaker 2 and the valve 6 closes by sealing against the edge of the drinking nozzle 7. Optionally, the valve 6 is provided with a sealing ring to this end. The valve 6 is connected to a shaft 11 by a first lever 9, the shaft 11 is rotatably attached between two ribs 20 which are attached to the top of the lid 4. A second lever 13 is also attached to the shaft 11, which second lever 13 can be moved upward by a compression arm 14. When the compression arm 14 moves the second lever 13 upward, the shaft 11 rotates and, as a result, the first lever 9 moves downward with the valve 6 and the drinking nozzle 7 connects to the beaker 2, following which the drinking nozzle 7 can be used to drink from. The valve 6 is designed such that it can be opened over a distance of for example 3-4 mm, so that solid ingredients, such as vegetables or meat in soups, present in the liquid, do not block the valve. During drinking from the drinking aperture 8 of the drinking nozzle 7, air may be sucked in through an opening 10 at the same time as liquid is drawn in from the drinking aperture 8. Consequently, it is more convenient to drink hot liquids, such as tea or soup, from the beaker 2, as the hot liquid cools down by the liquid from the beaker 2 and air being simultaneously sucked in through the drinking aperture 8. The air supply opening 10 is also used by the first lever 9 and is dimensioned such that the shaft 11 and the second lever 13 can be mounted through it. The compression arm 14 forms part of an operating arm 22 by means of which the valve 6 is operated. The operating arm 22 is provided with an opening 19 through which a shaft 18 is fitted, which shaft 18 is mounted in the ribs 20. The opening 19 is formed such that the shaft 18 can be brought into two limit positions. In the first limit position, as illustrated by solid lines in figure 1, the operating arm 22 is completely between the ribs 20 which are arranged on the lid 4 and between ribs 21 which are arranged on the side of the beaker 2. By pressing on the part of the operating arm 22 which is between the ribs 21, the operating arm 22 rotates about the shaft 18 and the compression arm 14 moves the second lever 13 upward and opens the valve 6. In the second limit position of the operating arm 22, as illustrated by a broken line in figure 1, the operating arm 22 is moved from between the ribs 21 and it is possible to rotate the beaker 2 relative to the lid 4 and to unscrew the lid 4 from the beaker 2. A plug 16 is arranged on the underside of the compression arm 14, which plug 16 is made of a flexible material. When the valve 6 is closed, the plug 16 closes an opening in the lid 4. When the operating arm 22 is activated, the compression arm 14 and the plug 16 are lifted up from the lid 4, as a result of which the opening under the plug 16 is unblocked and air can flow through it. This air supply makes it easier for the liquid to flow out of the drinking nozzle 7. The valve 6 is pulled toward the underside of the drinking nozzle 7 by means of a resilient element. In the embodiment shown, the resilient element is a rubber ring 15 which stretches between a hook 12 fitted to the second lever 13 and a hook 17 fitted to the operating arm 22. As a result of a tensile force of the rubber ring 15 on the operating arm 22, the latter is pulled against the shaft 18, the opening 19 being designed such that the shaft 18, as shown in figure 1, can assume two stable positions in the opening 19. The opening 19 can also be designed as a slotted hole through which the operating arm 22 is pulled in the first limit position by the rubber ring 15 and has to be held in the second limit position by the user when screwing the lid 4 onto the beaker 2 or unscrewing it. During use of the closable beaker 1, the beaker 2 is filled with a liquid, the operating arm 22 is brought into such a position that the lid 4 can be screwed onto the beaker 2, the lid 4 is screwed onto the beaker 2 and the operating arm 22 is moved between the ribs 20 and the ribs 21. Should the closable beaker 1 fall over now, no liquid will run out of it. The operating arm 22 will not be activated either when the beaker falls over, as it is protected by the ribs 20 and 21. Even if the liquid in the closable beaker 1 is carbonated, the valve 6 will remain closed and no liquid will run out. The user will grasp the beaker 2 with one or both hands when he/she picks up the closable beaker 1 and is able to operate the operating arm 22 with his/her fingers as a result of which the valve 6 can be opened. In addition to the embodiment shown in figures 1- 3, where the operating arm 22 is situated between the ribs 21 alongside the beaker 2 in the longitudinal direction, other embodiments are also possible. For example, it is possible to arrange the operating arm halfway around the beaker 2, with protecting ribs being fitted along the edges of the operating arm. This embodiment makes it easier to operate the operating arm using the fingers, making this embodiment particularly suitable for people with small hands. In this case as well, the protecting ribs along the edges of the operating arm ensure that the valve is not opened as a result of the operating arm being pushed if the closable beaker falls down. Figures 4-7 show a second embodiment of the closable drinking beaker 1. The main difference with the first embodiment is that the valve 6 is operated pneumatically. To this end, a supporting plate 23 is attached to the second lever 13, the shaft 11 and the supporting plate 23 being placed between the ribs 25. An air cushion 26 is arranged between the supporting plate 23 and the top of the lid 4. The air cushion 26 is connected to a bellows 29 by means of an air pipe 27. When the bellows 29 is compressed, the air cushion 26 presses the supporting plate 23 upward, thus opening the valve 6. When the bellows 29 is released, a spring 24, which is held between the ribs 25, presses the supporting plate 23 down and closes the valve 6. The air pipe 27 is held between the ribs 28, which run along the side of the lid 4. The bellows 29 is held between and under a rib 31 and can be released in a simple manner so that the lid 4 can be unscrewed. An opening 30 may be arranged in the bellows 29, which can be closed by the user with his/her finger when he/she compresses the bellows 29. Using the opening 30 prevents an unintentional buildup of pressure in the bellows 29 and the air cushion 30 when hot liquid is poured into the beaker 2. The opening 30 is not required if this buildup of pressure does not occur or only occurs to a limited degree. Use of the ribs 25, 28 and 31 prevents, inter alia, the bellows 29 from being compressed when the closable drinking beaker 1 falls down and the air pipe 27 from being damaged. In another embodiment, the bellows is located in a groove arranged around the beaker and when the closable beaker is picked up, the fingers grip the bellows and the valve is activated. In the exemplary embodiments shown above, the beaker 2 is of conical design, with the section of the beaker 2 being more or less circular and the operating arm 22 or the bellows 29 being arranged on the outside of the circular section. In another embodiment, this section is adapted to the shape of a hand to a greater degree, with the beaker 2 being of a more narrow design at the location of the operating arm 22 or the bellows 29, so that it is easier for the person wanting to drink from the closable drinking beaker 1 to grip around the beaker 2. Figures 8a-8d show a third exemplary embodiment. In this exemplary embodiment, the side of the beaker 2 is provided with a cylindrical edge 39 which ends in a conical edge 38. The cylindrical edge 39 is slid into a cylindrical lid edge 45 until the conical edge 38 is sealingly held against a seal 46, which forms part of the lid 4. The dimensions of the cylindrical edge 39 and the cylindrical lid edge 45 are chosen such that the lid 4 is clamped securely on the beaker. The lid 4 is provided with a guide blade 49 on one side and a support blade 50 on the other side in order to make it easier to slide the lid 4 onto the breaker 2 and to remove it. The guide blade 49 and the support blade 50 make it easier to slide the beaker 2 into the lid 4, as the cylindrical beaker edge 39 is guided to the cylindrical lid edge 45. The ribs 20 are arranged on the outside of the lid, along the support blade 50, between which ribs 20 an operating arm 22 is placed. The support blade 50 has an opening 41 at the bottom, into which hook 42 can be fitted. The operating arm 52 has a pressure surface 43 which can press on a projection 35 of a lever 37 and presses the lever 37 down in a manner to be described below. In order to make this possible, the hook 42 is designed in such a manner that it cannot move upward into the opening 41, so that the operating arm 52 can exert a downward force onto the projection 35. The lid 4 is provided with a drinking nozzle 7 having a drinking aperture 8 and an air supply opening 10 in a manner described above. By sucking in air through the air supply opening 10 during drinking from the aperture 8, hot liquid is cooled by means of the air which is sucked in. The drinking nozzle 7 can be closed by means of the valve 6 which is moved by the lever 37. The lever 37 is hingedly connected to the inside of the lid 4 by a hinge coupling, which in this embodiment is formed by shaft supports 33 into which shaft journals 36 can be fitted. The lever 37 is provided with a spring element 40 which ensures that the valve 6 seals the drinking nozzle 7 when the spring element 40 presses against the inside of the beaker 2 when the lid 4 has been placed on the beaker 2. When the spring element 40 presses the lever 37 toward the center of the beaker 2, the lever 37 turns in the hinge coupling and presses the valve 6 upward, as a result of which valve 6 closes the drinking nozzle 7 and, at the same time, an aeration opening 32 in the lid 4 is sealed by the projection 35. Optionally, the projection 35 may be provided with a sealing edge 44 in order to improve the seal. As mentioned above, the projection 35 interacts with the pressure surface 43 of the operating arm 52 and the operating arm 52 can press the projection 35 downward through the aeration opening 32, as a result of which the lever 37 moves down and the valve 6 opens. The support blade 50 has an opening 34 which is dimensioned such that the spring element 40 can extend through the support blade 50, as a result of which the lever 37 can rotate downward when the lid 4 is not located on the beaker 2 and the top of the valve 6 is readily accessible for cleaning when the lid 4 with the lever 37 is placed in a dishwasher. The lever 37 has a bottom edge 47 which is formed in such a manner that when the lid 4 is placed on the beaker 2, the spring element 40 is guided along the inside of the beaker 2 and the beaker 2 ends up between the spring element 40 and the support blade 30 and thus moves the lever 37 to the center of the beaker 2 and the valve 6 moves upward and closes. As described above, the valve is opened by the operating arm 52 being pressed toward the side of the beaker 2, as a result of which the pressure surface 43 slides past the projection 35 and presses this projection 35 inward. Consequently, the lever 37 and the valve 6 are pressed down and the drinking nozzle 7 can be drunk from. At the same time as the projection 35 is pressed down, the sealing edge 44 unblocks the opening 32, as a result of which air can flow into the beaker and drinking from the drinking nozzle 7 is facilitated. The lid 4 and the support blade 50 attached thereto are provided with ribs 20 between which the operating arm 52 is located. In the situation where the drinking beaker 1 is closed, the operating arm 52 is in any case partially between the ribs 20, so that the operating arm 52 is not activated when the drinking beaker 1 falls down, and the drinking beaker 1 remains closed. If desired, part of the ribs 20 may be less high along the side of the beaker 2, for example near the support blade 50, as indicated in the figure, so that the operating arm 52 can be pushed in more easily by a hand which grips around the beaker 2. The beaker 2, the lid 4 and the operating arm 52 are made of strong plastic, which is suitable for absorbing the forces which occur and which can readily be cleaned in a dishwasher. The lever 37 may be made of the same plastic, more flexible edges or rings being provided at the location of the valve 6 and the sealing edge 44. In another embodiment, the lever 37 is entirely made of a more flexible, rubber-like plastic. In order, in that case, to achieve sufficient strength, a metal insert, such as a small spring-steel plate (not shown) , may be placed in the lever 37, which insert passes the closing force of the valve 6 on to the hinge coupling with the shaft journals 36 and to the spring element 40. As is shown in figure 8b in detail, the hinge coupling is designed with shaft supports 33 having a through-hole 53 into which the shaft journal 36 fits with ample play. Preferably, the hole 53 is shaped in such a way that the shaft journal 36 is pressed into a predetermined position by the spring element 40 when the lid 4 is placed on the beaker 2. Consequently, the position of the shaft journal 36 is precisely defined during use and the lever 37 will move in the desired way during opening of the valve 6. When the lid 4 is open, there is sufficient space around the shaft journal 36 in the hole 53 to enable easy cleaning, for example in a dishwasher. As the hole 53 runs through the entire thickness of the shaft support 33, the shaft journal 36 can also be cleaned on the side remote from the lever 37 and thus on all sides. Figure 9 shows the lid 4 of a fourth embodiment with a number of modifications relative to the third embodiment. In the fourth embodiment, the lever 37 is designed with a more or less convex valve 6, so that no liquid stays behind on the valve 6 during use. In order to guide the conical top edge 38 of the beaker 2 towards the seal 46, use is made in this embodiment of guide cams 48 along which the cylindrical beaker edge 39 can slide. It will be clear to those skilled in the art that such guide cams 48 can also be fitted along the exterior on the side ι of the beaker 2, these guide cams being able to slide along the cylindrical lid edge 45. Figure 10 shows an alternative embodiment of the lever 37. In this case, the lever 37 is designed as a spring ring 51 which, in its unloaded state, has a curved profile, in such a manner that when the spring ring 51 is clamped between the beaker 2 and the lid 4 and pressed flat, the valve 6 and the projection 35 are pressed upward and seal the openings in the lid 4. In this case, the valve 6 is opened by pressing on the projection 35 in a similar manner to that described above. The advantage of this embodiment is that the lever 37 can easily be cleaned as a separate part. It will be clear that the spring ring 51 may be designed as a spring-steel plate provided with a layer of flexible plastic on top. If desired, the spring ring 51 may be made of strong plastic with separately fitted flexible material at the location of the seal, or of plastic with one or more preferably metal inserts in some places.