When you are at home, look under the sink to see the pipework. Do not touch any of the pipes.

If you look carefully you will see two thin pipes that lead up to the taps. These are clean water, one hot water, one cold.

Then, under the bottom of the sink is a larger pipe. This carries the waste water away.

You will probably see that this pipe has a bend in it. That is for health reasons because the waste pipe leads to sewers and we don't want any nasty smells come up the pipe. Water trapped in the end stops that happening.