Pour water into an ice-cube tray. Fill to just below the top edge -- the water will expand as it freezes.
Set a freezer to 0˚C or 32˚ F, which is the temperature at which water freezes.
Place the ice-cube tray on a flat surface inside the cold freezer.
Wait several hours, giving the water enough time to change from its liquid state to a solid state.
Remove the ice-cube tray from the freezer. The temperature will have converted the water molecules into solid molecules in the form of ice.