Here's why:

* Polar Head: Phospholipids have a phosphate group attached to a glycerol molecule. This phosphate group is hydrophilic (attracted to water), making it the polar head of the molecule.

* Non-polar Tail: Attached to the glycerol molecule are two fatty acid chains. These chains are hydrophobic (repelled by water), forming the non-polar tail.

This structure is fundamental to cell membranes. The phospholipids arrange themselves in a bilayer with the polar heads facing the watery environments inside and outside the cell, while the hydrophobic tails face each other in the interior of the membrane. This structure creates a barrier that controls what can enter and leave the cell.