Tell a joke about the electric switch.

The son who went to kindergarten heard that his father had prostate problems, and the advertisement on TV said: The reason for prostate problems is that the faucet can't be turned off. The son asked his mother: Why is the faucet easy to turn off? As soon as the light was turned off, it went out. Mom said: the faucet uses the principle of blocking; The electric switch uses the disconnection principle. You will understand later. One day, my son and his mother went shopping together. He insisted that his mother buy an electrical switch. Mom asked: Why buy it? The son said: this is a man's business, so leave it alone for you women. Mom smiled and said, well, I don't care. I'll buy you one. After returning home, the son happily took the switch to find his father and mysteriously said to his father, put it on and your prostate problem will be solved!