So, we were at this 3-day school camp, some time ago, and 6 kids share a bedroom. So, it was almost eleven-o'clock at midnight, and yet we still couldn't get to sleep, yet this one kid had already fallen asleep and was even snoring a bit. So me and my two best friends told everyone about this prank which we were going to do, and they all joined in. We woke up the kid, and told him that it was already the end of the second day of the camp, and that he had slept through the
entire day, missing out on all the activities we did. At first it was a pretty lame joke, he did not buy it, but then
everyone joined in about there favorite activity, and this made him uneasy a bit.
There are these adults that patrol the corridors to make sure that kids actually fall asleep and not spend the night talking, and one of them came in and wasn't too happy with us. Then someone who I don't even remember asked if it was the second day (which it obviously was). The person said it was, of course. THAT set him off, and then he started to pack his bags and swear at us and the adult was absolutely baffled by the incident.
This is the only prank which I can remember for now...

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