A physical phenomenon known as osmosis
causes water to flow from an area of low solute, high water
concentration to an area of high solute, low water concentration, until
the two areas have an equal ratio of solute to water. Normally, the
solute diffuses toward equilibrium as well; however, all cells are surrounded by a lipid bilayer
cell membrane which permits the flow of water in and out of the cell
but restricts the flow of solute under many circumstances. As a result,
when a cell is placed in a hypotonic solution, water rushes into the membrane, increasing the cell's volume. There you go! Have fun