Hydrophobic interations occur between 2 or more nonpolar molecules when they're in polar environments (most commonly Water). Their 'dislike' to water causes the molecules to stick together or fold in a certain way, in order to interact with the polar environment as little as possible. Some examples of hydrophobic interactions include the folding of the tertiary structure in proteins and the specific double helical structure of DNA. There are also hydrophobic interactions within clusters in amphipathic/amphiphillic molecules, such as the phospholipid bilayer in membranes. Hydrophobic interactions occur in amphipathic molecules in order to stop the water molecules' ability to surround them, the hydrophillic regions are orientated in a way that allows them to act as a protective outer structure because it interacts amicably with water. Phospholipid bilayers have numerous molecules with a hydrophilic head on the outside and a hydrophobic tail. These molecules lay "back to back" forming a hydrophoic centre of tails and two outer layers of hydrophilic heads.