What is funny?

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What are some of the thing that make you laugh? What is your own personal definition of "funny"? One of the things I find hilarious is when people get scared suddenly, like when somebody pops into view and shouts "boo!" at you. The look on a person's face right when they jump is hilarious. xD And I think whe someoone farts really obviously and acts like nothing happened is really funy, too. xD