According to Osmosis and Diffusion, when a cell is placed in a Hypotonic Solution, it has a lower concentration of water molecules than its surrounding environment. So, the concentration gradient allows the molecules outside the cell to diffuse inside it. This is because during Diffusion, molecules move from a High Concentration to a Low Concentration.
Btw, I am in 9th grade and I am taught this and AP Biologuy work because my teacher over teaches us.