Lipid bilayers

Picture a sandwich, but instead of bread, it is made of two layers of tiny, tiny, very thin pieces of fat. These layers are called a "lipid bilayer." The fatty pieces are called lipids, and they are arranged so that their water-loving (hydrophilic) heads face outwards, and their water-fearing (hydrophobic) tails face inwards, away from the water.

Just like a sandwich, things can get stuck in between the two layers of the lipid bilayer. These things are usually molecules like proteins, which help the cell do important things like communicate with other cells or transport nutrients.

The lipid bilayer is pretty special because it's what separates the inside and outside of a cell. It's like a protective wall that keeps harmful things out and lets good things in. But it's also flexible, so the cell can change its shape and move around.