Adsorption
Adsorption is when atoms or molecules stick to the outer surface of a solid instead of soaking in.
Think of it this way
Dust sticks to a wooden shelf the way adsorption works. The dust settles on top of the wood — it does not soak in. Each tiny dust particle clings to the outer surface and stays there. The shelf holds the dust on its face, not inside it.
Explaining adsorption by grade level
Think of dust on a table. The dust sits on top of the table. It does not go inside the wood. Charcoal works the same way with bad stuff in water. The bad parts stick to the outside of the charcoal.