Why would a cell lyse when it's in water?

Lysis, or the bursting of a cell, happens because of a cell swelling excessively causing it to burst, due to the movement of water into the cell by osmosis. However, this isn't the case for all cells!

Osmosis is the movement of water from an area of high water potential, to an area of low water potential, down a concentration gradient, across a semi permeable membrane. If a cell is placed in a solution or solvent that has a higher water potential of the cell's cytoplasm, the water would rush into the cell by osmosis: if a cell's cytoplasm, e.g. a red blood cell, has a water potential of -300 kPa (this number isn't representative) and the water is placed into has a water potential of 0 kPa (which is the value for pure water), the red blood cell would burst as the water would cross into the cell across the red blood cell's membrane causing it to swell up and, eventually, burst. As the water has a higher water potential than that of the cytosol, the water would be known as a hypotonic solution, which is a term that describes the relation of the "concentration" of water molecules in the solution (which in this case is pure water) compared to that of the "concentration" of water molecules in the cell cytosol (which in this case is the red blood cell's cytosol, which would contain water as well as lots of dissolved salts and other things, which "lowers" the water "concentration" of the cytosol, which is why the cytosol's water potential is relatively low).

In this case, the red blood cell bursts as the cell membrane is quite flimsy and weak, think of a balloon! However, if this were a plant cell, the cell would not burst. This is due to the strong cell wall that surrounds the cell, made out of the polysaccharide cellulose, which has a high tensile strength and would be able to hold the cell and prevent it from bursting.