When an animal cell is placed in a hypotonic solution it will

Gain water due to osmosis moving water inside of the cell

An animal cell that is placed in a hypotonic solution will rapidly gain water, because osmosis would cause the water to move to an area with more solutes. In this case, that is the inside of the cell. A cell in a hypotonic solution may gain enough water to lyse, or rupture, the cell membrane, which destroys the cell.