Those times when you use your bathtub to heat your sink. A lot of times when one washes their hands at the sink and turns the water on hot, nothing but cold water comes out of the faucet. By letting the water in the bathtub run for about 10-15 seconds, the water heats up. Then you turn off the water in the bathtub, run your water on hot at the sink, and in a couple of seconds, you have hot water running at the sink.
Normally when the Bathtub Sink Effect is not done, you can run the sink water for minutes and get nothing but cold water even though you have it on hot.