The less movement we have, the more rigid that membrane is. Now, if the fluidity is determined by the relative movement, what determines the relative movement itself? Well, the relative movement of the molecules within the membrane is determined by the strength of the attractions, the non covalent intermolecular bonds that exist between the phospholipids, the molecules in the membrane. So you might imagine that in a membrane in which we have more fluidity, we have more movement. And what that means is the attractions are not very strong. But in a membrane in which the attractions between the molecules is strong, that means we're going to have less movement.