When an animal cell is placed in a hypotonic solution, the concentration of solutes outside the cell is lower than that inside the cell. As a result, water from the surrounding environment tends to move into the cell by osmosis.
This influx of water causes the cell to swell as it takes on more water. If the solution is significantly hypotonic, the cell may continue to absorb water and eventually reach a point where it can no longer maintain its structure. If too much water enters, the cell could burst, a process known as cytolysis.
In a nutshell, placing an animal cell in a hypotonic solution leads to swelling and potential bursting due to the excess water entering the cell.