Here's why:

* Phospholipids: These are the primary building blocks of the cell membrane. They have a unique structure with a hydrophilic (water-loving) head and hydrophobic (water-fearing) tails.

* Bilayer: These phospholipids arrange themselves into two layers, with the hydrophilic heads facing outwards towards the watery environment both inside and outside the cell, and the hydrophobic tails facing inwards, creating a barrier.

This arrangement creates a selective barrier that regulates what enters and exits the cell, acting as the gatekeeper for the cell.

Important Note: The phospholipid bilayer is only part of the story. Other components like proteins and cholesterol also play vital roles in the cell membrane's functions.