The Black family is going to London for a holiday today. And they have to be at the airport at eleven forty. "It'll take us thirty minutes to get there in the taxi," Mr Black says. "So we have to be ready by eleven o'clock. Nobody should be late. "

At ten to eleven, Mrs Black is sitting quietly in the garden. But Mr Black and their son and daughter are still running around in a hurry. At eleven o'clock, the taxi gets to their home. Mrs Black says to them, "Well, I know that this is going to happen, so before I went to bed last night, I moved all our clocks and watches forward（向前拨动）by twenty minutes. Now we can go to the airport without worrying about being late. "