Osmosis

Osmosis is like the way a sponge soaks up water. Your body is made of cells, and those cells are like tiny sponges. All around the cells, there is a liquid called 'water'. Sometimes, different amounts of certain stuff, like sugar and salt, are in the water.

Your cells have special membranes, like a wall, that only lets certain stuff in or out. When the stuff in the water outside your cells is different than what's inside your cells, the liquid will slowly pass through your cell membrane, into the cell, until it's even on both sides. This process is called osmosis.