Cell Membrane
Cell Membrane is the thin covering around a cell that controls what goes in and out.
Think of it this way
A kitchen strainer sits over a bowl and lets water pass through but keeps pasta inside. The cell membrane works the same way. It is a thin layer around the cell that lets some things pass in or out but blocks others. Water and small nutrients can cross through, but larger or harmful things cannot.
Explaining cell membrane by grade level
Every cell has a thin skin around it. This skin lets good things in and keeps bad things out. When you soak an egg in salty water, water moves through that skin. The egg gets smaller because water leaves.