All cells have a cell membrane which is described as being "selectively Permeable". this means that some materials can move easily in or out of the cell through the cell membrane as though it were a screen. When a substance passes through the membrane without any help from the cell,it's most likely caused by diffusion. Water is a substance that can do this. When water diffuses into or out of a cell it is called "Osmosis". Since calls have water in them and basically live in water-based environment,osmosis is always possible