And so, because we don't want to have high electrostatic repulsion, we don't want to waste so much energy, our phospholipids will only move back and forth and will not jump vertically, as shown. And this type of motion, a constant fluid like motion of our phospholipid bilayer, is known as the fluid mosaic model. Now, the last part that I want to discuss is an important part of our phospholipid bilayer, that is our cholesterol. So cholesterol is another important type of constituent that is found inside our cell membrane. And what cholesterol does, it basically controls the fluidity of our cell. So how fluid or how rigid our cell is?