Why does the hot water slow to a trickle after initially blasting out?

Dear Cecil: I have an old apartment complete with old pipes and I’ve noticed an annoying phenomenon when I turn on the hot water. At first it comes blasting out, but within a short time it slows to a trickle, as though a poltergeist were under the floor squeezing the pipe. What gives? The pipe stays the same and I assume the water pressure does too. Is there some law of fluid dynamics that causes this? It doesn’t seem to happen to cold water pipes. Mike P., Dallas, Texas

Cecil replies:

Cecil would love to tell you there’s some mind-boggling phenomenon at work here involving gravitons and the strong nuclear force, but, unfortunately, no can do. According to my Plumbing Repairs Made Easy — I love books with titles like this — the hot water slows to a trickle because the washer in the faucet expands when it gets hot. Solution? “Replace with a proper, nonexpanding washer.” Short, simple, and unlikely to inspire much backtalk from the troops. Sometimes I think I went into the wrong line of work.

Cecil Adams

Send questions to Cecil via cecil@straightdope.com.