My favourite - No. 2 You have seen someone take a shower in class.

 ... which reminds me of a student who, out of curiosity, decided to pull the hanging shower handle in the chemistry lab and got himself all wet. Needless to say, he had to forgo his recess to mop the (very) wet floor.

There was a positive outcome though - we finally verified that the never-been-tested safety shower in that lab worked perfectly!

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P.S. If you don't know what a safety shower is, the following video will show you the importance of such showers and eye-washes in a chemistry laboratory.