A COOLING DEVICE [001] This invention relates to a cooling device incorporating shelves whereby efficient use of the internal volume is achieved. [002] In cooling devices, preferably in refrigerators, to be able to place tall food and beverage containers, one or more of the shelves are detached from their places or then- positions are changed. In the state of the art, foldable or telescopic shelf systems are utilized in order to form a flexible usage volume enabling containers with various dimensions to be preserved. That type of shelf systems should be in a form in harmony with the appearance, functional and safe at the same time. [003] In the British Patent Document GB 1191596, a description is given of a shelf, utilized in a refrigerator, with a supporting surface in form of spaced slats wherein springs are incorporated and the said shelf may be pushed along the depth of the re¬ frigerator, from the left or right. [004] In the United States Patent Document US 3264049, the shelves used in the re¬ frigerator comprise an extensible shelf support that rests on the subjacent shelf by means of a leg structure in order to create an additional space inside the refrigerator in vertical direction and for the purpose of flexible usage. [005] In the United States Patent Document US 3625371, a description is given of a shelf structure for use in a freezer, incorporating a panel that is openable and closeable in vertical direction. [006] In the United States Patent Document US 2319 Al Q, a description is given of a shelf construction utilized in a refrigerator, comprising a main shelf supported by the walls, an additional shelf attached to the main shelf by means of a hinge structure, a second additional shelf connected to the said additional shelf and supporting elements where additional shelves rest on when they are folded upwards or downwards. [007] In the French Patent Document FR 2637057, a description is given of a shelf structure which comprises two openwork shelves, foldable in vertical direction, which are connected to one another by means of double hinges and which can move in various slideways inside the refrigerator. [008] In the British Patent Document GB 0937482, a description is given of an ap¬ plication concerning a foldable shelf used in a refrigerator wherein two half portions of the said shelf are connected to each other by a hinge at a joint extending from the front to the back and, permit the shelf to fold downwards if and only if the afore-mentioned hinged joint moves upwards. [009] The aim of the present invention is the realization of a cooling device comprising shelves that can fold easily and safely, ensuring that the internal volume is utilized ef- fectively. [010] The cooling device realized in order to attain above mentioned aim of the invention is illustrated in the attached figures where: [011] Fig.1 - is a perspective view of a cooling device. [012] Fig.2 - is a front schematic view of a cooling device. [013] Fig.3 - is a top view of a foldable shelf. [014] Fig.4 - is a schematic view of the stationary and foldable portions of a shelf, wherein they are not mounted. [015] Fig.5 - is a schematic view of a shelf, unfolded. [016] Fig.6 - is a schematic view of a shelf, folded. [017] Fig.7 - is a schematic view of a folded shelf and hinges. [018] Fig.8 - is a schematic view of hinges and a shelf the foldable portion of which is detached. [019] The elements illustrated in the figures are numbered individually as follows: 1. Cooling device 2. Shelf 3. Shelf groove 4. Main shelf 5. Movable shelf 6. Hinge 7. Extension 8. Rest surface 9. Extension housing 10. Handle 11. Pin 12. Support 13. Pin housing 14. Assembly gap 15. Main shelf support 16. Movable shelf support [020] The cooling device (1) comprises one or more than one foldable shelf (2) wherein several items are placed on and one or more than one shelf groove (3) where the shelves (2) can move inside, carrying the mentioned shelves (2) and holding them at a desired level. [021] The shelf (2) comprises a main shelf (4) forming the stationary portion, a movable shelf (5) forming the foldable portion, attached to one side of the main shelf (4) and which can move downwards about the rotation axis constituted by its side attached to the main shelf (4) and, one or more than one hinge (6) whereby the movable shelf (5) folds and is attached to and detached from the main shelf (4). [022] The main shelf (4) comprises an extension (7) along the side of which the movable shelf (5) is attached to, whereby it is achieved to obtain a continuous coplanar surface on the shelf (2) in unfolded position and to hide the hinges (6) in both folded and unfolded position and, a rest surface (8) wherein the movable shelf (5) stands vertical by resting on it, in folded position. [023] The movable shelf (5) comprises an extension housing (9) along its side attached to the main shelf (4), whereby it is achieved that the extension (7) fits in when the movable shelf (5) and the main shelf (4) are brought to the same plane, and that the shelf (2) stays in a straight position and is prevented from folding upwards by supporting it at the area of folding and, a handle (10) whereby the said movable shelf (5) is folded easily, brought to a straight position or, if desired, taken out of the cooling device (1) by being detached from its position. [024] The hinge (6) comprises a pin (11) functioning as a rotation axis where the movable shelf (5) can move about, a support (12) placed on the main shelf (4) or the movable shelf (5), where the pin (11) is fixed to, a pin housing (13) with a size allowing the pin (11) to rotate, which is positioned on the main shelf (4) or the movable shelf (5) and where the pin (11) fits into, and an assembly gap (14) the length of which is larger than the length of the pin (11), whereby pins (11) and pin housings (13) are aligned as the movable shelf (5) is attached to or detached from the main shelf (4). [025] The movable shelf (5) can be taken out of the cooling device (1) by being detached easily from the main shelf (4) by the user, in folded or unfolded position, and can be attached to the main shelf (4) again in the desired position. In the preferred <■ embodiment of the present invention, pins (11) and supports (12) where the said pins (11) are connected to, are positioned on the main shelf (4) whereas pin housings (13) are positioned on the movable shelf (5). Thereby, pins (11) and supports (12) stay on the main shelf (4) that is not taken out of the cooling device (1) and are hidden from the top by the extension (7) and, the movable shelf (5) can be taken out of the cooling device (1) more safely on the side of the user since only pin housings (13) are positioned on it. [026] The cooling device (1) which is the object of the present invention, comprises one or more than one main shelf support (15) positioned on the rear wall of the cooling device (1) and/or on the main shelf (4), whereby the main shelf (4) stands in balance when the shelf (2) is folded and, one or more than one movable shelf support (16) positioned on the rear wall of the cooling device (1) and/or on the bottom surface where the movable shelf (5) rests on, which facilitates, in folded position, the movable shelf (5) to stand vertical and to support the main shelf (4). [027] The form of the side of the shelf (2) ensures that the movable shelf (5) can easily detach from the shelf grooves (3) inside the cooling device (1) where it is attached to, if it is to be folded and, that the movable shelf (5) rests on the bottom surface after it is folded. [028] As the shelf (2) is unfolded, a rigid structure is obtained due to the fact that the extension (7) fits into the extension housing (9) on the movable shelf (5) and so it is not necessary to support the shelf (2) additionally at the area of folding. The sides of the main shelf (4) and the movable shelf (5) which are connected to each other, appear as a thin line on the top surface of the shelf (2) and do not prevent the food containers from changing their position on the shelf (2). [029] As the movable shelf (5) is folded downwards by grasping the handle (10), it bears the rest surface (8) and stands vertical. Taller food and beverage containers may be placed to the space (A) which is formed near the shelf (2) as the movable shelf (5) is folded and furthermore, a compartment (B) with a closed side is formed under the main shelf (4). [030] In folded or unfolded positions of the movable shelf (5), it is ensured that the hinges (6) are left under the shelf (2) by being covered with the extension (7) and thereby the appearance is improved. Through the use of the foldable shelf (2) flexible use of the internal volume of the cooling device (1) by the user is achieved while safety re¬ quirements are satisfied and aesthetic appearance is not disturbed.