O que significa April fool ?

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There is a tradition that the first day of April each year is a day when we play tricks on each other. The person that is being tricked is called the April Fool. After we play the trick on someone, instead of saying "I was only joking" or "I was playing a trick", we just say "April Fool". Then the person knows that it was all a trick. It is all in fun and no one gets angry (usually). If you put ping-pong balls in someone's shoes so that they can't get their shoes on, or if you set someone's clock ahead and then convince them that they are late for work, those are April Fool tricks. You only trick your friends and family, and there can be a of of rivalry.