Deformation
Deformation is when something changes shape because a force pushes, pulls, or crashes into it.
Think of it this way
When you press your thumb into a ball of bread dough, the dough changes shape under the force. The spot where you push sinks in, and the sides bulge outward. Release the pressure and the dough slowly springs back — or stays dented if you pushed hard enough. The force from your thumb caused the deformation.
Explaining deformation by grade level
Push a paper cup down hard with your hand. The cup bends and gets squished. That new shape stays because you pressed too hard for the cup to bounce back. When you crash something into a barrier, the same kind of shape change happens.