Penetration Depth
Penetration depth is how far an object pushes into a material before stopping.
Think of it this way
Push a knife into a stick of butter. The blade sinks in a few inches, then stops. That distance from the surface to the tip is the penetration depth. A dull knife in cold butter stops early. A sharp knife in soft butter goes much deeper.
Explaining penetration depth by grade level
Push a nail into clay. A heavy hammer pushes it deep. A light tap leaves it near the top. Heavier force means the nail goes further in. That deeper push is penetration depth.