And that's because the individual constituents, the molecules that make up the membrane, are in a constant state of motion. So we have two types of motion. Obviously, we discussed lateral motion or lateral diffusion. And what that means is if we take a look at the membrane so we have two sides of the membrane, two leaflets, one leaflet and the posing leaflet. So the individual fossil lipid molecules, these lipid molecules along any leaflet can move relatively quickly and easily along that leaflet and they move at a rate of about one micro meter per second. Now, proteins within the membrane can also move along that membrane in a lateral direction.