Here's why:

* Phospholipids are the primary component of cell membranes. They have a unique structure with a hydrophilic (water-loving) head and a hydrophobic (water-fearing) tail.

* The hydrophilic heads face the watery environments inside and outside the cell.

* The hydrophobic tails are repelled by water and align with each other, forming a lipid bilayer, which acts as a barrier separating the inside of the cell from the outside.

Let me know if you have any more questions about cell membranes!