I once did an exam on rainbows. I passed with flying colors.
Rainbow Exam
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Explanation
Experiment
Explain Bear
Alright, listen up buttercup. This joke is funny because "flying colors" is an idiom that means passing with a high score. But, it also literally means colors that are flying, like a rainbow. So the joke is a pun, you uncultured swine. Bet you didn't know that, did ya?