Which characteristic makes a cell membrane selectively permeable?

Think about the building you're sitting in right now. What separates you from the outside world? The walls, right? They keep you safe from the elements, but they also let certain things in or out. In a classroom, the doors let students into the right classes, but can keep students out that belong somewhere else.
Let's look at how this translates to cells. Cells are the basic units of life that make up all living things. Cells, just like the rooms they are named for, have an outer barrier to keep the good stuff in and keep the bad stuff out. This barrier is called the cell membrane. The cell membrane has tiny gates that act like doors and windows in a room. This property of only letting certain things in and out is called selective permeability. Let's look closer at what this means.