Membrane fluidity refers to the viscosity of the lipid bilayer of a cell membrane. Cell membranes are fluid, meaning they are not fixed in position and can adopt amorphous shapes. The fluidity of a cell membrane can increase by a number of factors. Shorter fatty acid tails will increase fluidity as they are less viscous and more susceptible to changes in kinetic energy. At higher temperatures, phospholipids have enough kinetic energy to overcome the intermolecular forces holding the membrane together, which increases membrane fluidity. Cholesterol acts as a bidirectional regulator of membrane fluidity. At low temperatures cholesterol intercalates between the phospholipids and prevents clustering to increase fluidity. Additionally, unsaturated tails have double bonds, and as a result, have crooked tails. Because there is more distance between the tails of unsaturated fatty acids, they in turn have fewer intermolecular interactions creating more membrane fluidity.