A suction cup is a device made of flexible material, typically rubber or plastic, that creates a vacuum seal when pressed against a smooth surface. This vacuum allows the suction cup to stick firmly to surfaces like glass, tiles, or metal without the need for adhesives. Suction cups are commonly used for hanging items, such as shower caddies or decorative hooks, and can be found in various sizes and shapes. The principle behind a suction cup's function is simple: when the cup is pressed against a surface, air is pushed out from underneath it, creating lower pressure inside the cup compared to the outside atmosphere. This pressure difference holds the cup in place. Suction cups are also used in automotive applications, such as mounting GPS devices or smartphones on windshields.