mine was we were on school camp and i had blood capsules in my mouth so i tripped(wink wink nodge nodge) and fake blood was poring every wher, the teacher started chasing me round the camp ground, she still hates me now.
those capsules are epic. it reminds me of those small stinkbombs that had were in glass capsules. nowadays they are forbidden but we used to have alot of fun with them in the past.
for example you simply glue two of those capsules to a micetrap and put them under the front seat in a teacher's car. you can never tell when they go off but let it be 1 day or a year they will go off. it is usualy followed by heavy cursing so covering your ears is adviced^^