All cells are surrounded by a cell membrane which is partially permeable
Water can move in and out of cells by osmosis
Osmosis is the diffusion of water molecules from a dilute solution (high concentration of water) to a more concentrated solution (low concentration of water) across a partially permeable membrane
In doing this, water is moving down its concentration gradient
The cell membrane is partially permeable which means it allows small molecules (like water) through but not larger molecules (like solute molecules)