Due to their amphipathic nature, phospholipids spontaneously assemble into energetically stable structures in aqueous environments. While their polar heads are attracted to water, their long, nonpolar tails are excluded from water and strongly attracted to one another. This causes the hydrophobic tails to congregate within enclosed areas, shielded from water by the hydrophilic heads. Common arrangements include micelles, lipid-bilayer sheets, and lipid-bilayer spheres.