(Warning: This node may contain spoilers)
In the Harry Potter novels by J.K. Rowling, the Whomping Willow is a variety of tree.
A Whomping Willow reacts in a highly violent manner to anybody who invades its space, thrashing its limbs ferociously. This means that approaching one is not a good idea for anybody who doesn't want to put his or her life in danger. However, you can go near it and live to tell the tale if you know the secret of how to calm it down -- there's a knot near the base of its trunk which, if pressed, will keep the Whomping Willow placid for a brief period. It's advisable to use a very long stick to push the knot, rather than trying to get close enough to reach it yourself, though animals (or humans with animagus powers) might be able to sneak up to it.
A Whomping Willow was planted on the grounds of Hogwarts School of Witchcraft and Wizardry during the time that Professor Remus Lupin (well, he wasn't a professor yet at the time) attended Hogwarts as a student, roughly 20 years before Harry Potter. As disclosed near the end of Harry Potter and the Prisoner of Azkaban (and hence a spoiler, as noted earlier), the tree was planted to conceal the secret passage used by Lupin to go to a house in the nearby village of Hogsmeade once a month, as he was secretly a werewolf and needed to go someplace where he wasn't in danger of harming anybody in his transformed state. This passage is one of several secret passages noted on the Marauder's Map.
Early in Harry Potter and the Chamber of Secrets, Ron Weasley crashes a flying Ford Anglia into the Whomping Willow while driving himself and Harry to Hogwarts after missing the Hogwarts Express train. They manage to survive the experience (as does the Willow, with some minor injuries), but the car decides at this point that it doesn't want anything more to do with crazy drivers that would put it in places like that, so after unloading its passengers and luggage, it escapes into the wilds of the Forbidden Forest, where it apparently makes its home ever since.
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