Here's why:

* Phospholipids are the primary component of the cell membrane. Each phospholipid has a "head" and two "tails".

* The head is hydrophilic (attracted to water) and faces the watery environments inside and outside the cell.

* The tails are hydrophobic (repelled by water) and face each other, forming the interior of the membrane.

This hydrophobic interior acts as a barrier, preventing water-soluble molecules from easily passing through the membrane.