So let's say this is the outside, this is the inside. And because we have a bilayer membrane, we have two layers. Let's call this the outer leaflet that points towards the outside. This the inner leaflet that point towards the side of plasma. Now, if we take take a line and we draw a line that separates the membrane in half, like so, we'll see that this side is not the same as this side. These two leaflets are not identical.