Final answer:
The tails of phospholipids are hydrophobic and contribute to their classification as amphipathic molecules, with the hydrophilic heads facing water and hydrophobic tails facing away. Double bonds in the tails increase membrane fluidity.
Step-by-step explanation:
The characteristic that describes the tails of phospholipids is hydrophobic. This is because the fatty acid tails of phospholipids are uncharged or nonpolar, and thus repel and are repelled by water. Phospholipids are classified as amphipathic molecules since they contain both a hydrophilic (water-loving) part, which is the phosphate group head, and a hydrophobic (water-fearing) part, which are the fatty acid tails.
In terms of what contributes to the fluidity of a membrane, the characteristic of phospholipids that plays a role is double bonds in the fatty acid tail. These double bonds create kinks in the fatty acid tails, preventing them from packing tightly together, which increases the fluidity of the cellular membrane.