Why is it that when you push two pieces of wood together, they don’t smush together and become one smaller piece of wood? Because the atoms that make up the pieces of wood push against each other, repel each other. They want to own their own little bit of space. They resist being smushed.
But they can only push back so hard. With enough force, you *can* smush atoms together, breaking them down into smaller things. Those smaller things also don’t want to be smushed together, but with enough force, they will too. Everything will, with enough force.
