The Geography teacher in our secondary school (I didn't do Geography - he was subbing for a different class) told us a story about when he was in school. One day, he and his friends snuck into his chemistry lab and took a lump of potassium (a highly sensitive metal - it will combust if exposed to moisture (or even humid air) for too long). As a joke, the plundering posse poured slices of it down a drain (potassium is also quite soft - easy to cut up). When they were finished, my teacher-to-be stuffed what remained into his pocket.
The thing is, it was quite warm that day. Whilst inside his classroom, he bagan sweating. Moments later, he felt a - honestly, I don't know how to say this in a less... suggestivly ambiguous manner - "burning sensation in his pants". It was the potassium; he forgot to dispose of it and now it was reacting to his sweat. Calmly, he stood up and proceeded to walk out the classroom. Bewildered, his teacher told him to sit back down, but he just continued to the hallway. When he thought no one was around - he dug his hand into his pocket and frantically threw out all the potassium.
Unfortunatly, before all of it was removed... it ignited. In his pants. Needless to say he got quite the nasty burn along his left leg and arm, and tiny blobs of potassium were scatterd along his left side, also bringing tremendous pain. To add insult to injury, his pants caught fire and he had to remove them in front of the huge crowd that inevitably gathered. He was rushed to hospital. The inklings of potassium that stuck to him wouldn't be removed for a month, which he assures us, was pure agony. But he recovered, of course.
Surprisingly, the school didn't punish him, even though they knew of the drain incident. He reckons they were acting nice so that he wouldn't sue the school, which he wouldn't have done regardless. He was aware of his own recklessness (this is especially true considering he knew exactly about the risks of messing with potassium - he was an avid chemistry student).
What a great story to share, eh?