Here's one of those things you shouldn't have to tell people . . . but apparently they NEED to be told. The next time you're messing around with firecrackers, don't light one and put it in your POCKET.
There's a video of some idiot in England who did it on a dare, and ended up with a huge welt on his leg . . . plus one less pair of shorts he can wear, because it completely DESTROYED one of the pant legs.
(Search for "Man Puts Powerful Firecracker in His Pocket." He lights it at 1:04. WARNING: Several F-bombs and other profanity.)