But which source would you say the Hitchhiker's movie was adapted from?
It began life as one of a series of radio plays penned by Adams, each of which was to end with the destruction of Earth; he found later that he wanted to follow the tale of Arthur Dent and Ford Prefect after the story "ended". He started work on the novel, interrupted it to do the radio plays, adapted the radio plays into recordings with some differences in the plotline (for instance, in the recordings when they were escaping Hotblack Desiato's stuntship, they used pods; the one Zaphod and Trillian got into was actually an alien being, which ate them), finished the novel, then the second novel, then did the second series of radio plays, then...
No two stories were alike, a fact in which Adams reportedly took great pride. He never let anything go unused, either - Dirk Gently's Holistic Detective Agency, for example, began life as a script for Doctor Who.
The worst film "adaptation" (which gets the quotes because they only decided they were "adapting" the novel when someone pointed out how similar parts of the script were to the novel), for my money, would be Starship Troopers. Paul Verhoeven wanted to make a semi-satirical Nazis In Space movie, and in the process crapped all over Heinlein's meditation on honor, duty, courage, and how a man should relate to his country. If he'd just stuck with his original story, Bug War, it could have been good - Robocop did a great job of satirizing corporate culture - but instead, he had to pretend to adapt a Hugo- and Nebula-Award-winning novel. Feh, I say, feh!
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Sodium is a metal that reacts explosively when exposed to water. Chlorine is a gas that'll kill you dead in moments. Together they make my fries taste good.