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Why is there a watermelon there?
Two answers from the Blue Blaze Irregulars fan club (way back in the eighties). We all received a newsletter that had an interview with the makers: 1) they said that they added that line to add a bit of humor to the chase scene and 2) they said that, in the reality of the movie, Buckaroo is always looking to help the poor without having to deal with governments, so the watermelon is being tested to withstand the forces of being air-dropped to the hungry. Can't have our watermelons exploding upon impact, can we?
In the DVD commentary, the director states that the watermelon was put in the scene to test whether the studio was actually looking at the dailies.