text: The concentration of water inside versus outside.  If there is a greater concentration of water outside, the water will stay into/come into the cell.  If the concentration of the water is greater inside, the water will try to escape to the outside.  Think of it this way, if you put too much air inside a balloon it will grow and grow until it's too full and the air has to escape, and it's similiar with water in a cell, if the cell gets too full the water will want to escape out to where it's not as full.