sentences:  The momentum of the water is suddenly transferred into the fitting and Newton's Third Law kicks in forming growing the high-pressure region of water as it all "piles up" in the pipe.This high pressure region then travels back along the pipe in the form of a wave.The border of the high-pressure zone is referred to as a pressure wave, or transient (which is apt as the general definition of a transient is something that only exists for a short period of time).