Why is pure water hypotonic?

Pure water is considered hypotonic because it has a lower concentration of solutes compared to the inside of most cells. When cells are placed in pure water, water molecules move into the cells through osmosis, aiming to balance solute concentrations. This influx of water can cause cells to swell and potentially burst if they take in too much water. Therefore, pure water's lack of solutes makes it hypotonic relative to cellular environments.