Here's why:

* Gatekeeper: It controls what enters and exits the cell, just like a gatekeeper controls who enters and exits a building.

* Selectively permeable barrier: It allows some substances to pass through while blocking others, much like a barrier that lets certain things pass through but not others.

The cell membrane is a thin, flexible layer that surrounds every cell. It's made up of lipids and proteins, which arrange themselves in a way that allows for this selective permeability.