What is a water hammer?
Water hammer is the often counterintuitive result of stopping water that is in motion in a fluid system; the collective momentum of the water acts as a "hammer" and can create a high-pressure shockwave that damages, or destroys piping, valves, or equipment.

For example, it may be easy to close a water valve. Yet moments later it might explode from the sudden pressure of all the "upstream" water being forced to stop.