When you bump one end of a long metal bar, the other end does not instantaneously move. The movement would be instantaneous if the bar were perfectly rigid, but perfectly rigid materials are fundamentally impossible in the real world. Although the movement of the bar may seem uniform and instantaneous to our human eyes, it is really not. Our human eyes are simply too slow to notice the quick, but non-instantaneous, sequence of events that happens.

Public Domain Image, source: Christopher S. Baird.When you bump one end of a bar, you only locally deform that end and the rest of the bar is unaffected at first. But by inwardly deforming the near end of the bar, you have created a region of high pressure in the bar surrounded by regions of lower pressure. Said another way, you have forced the atoms in the near end of the bar to get closer to each other than the equilibrium positions of their chemical bonds. Said more simply, you have knocked the first few layers of atoms against the next few layers of atoms. What happens next?