Why are student pilots' shirt tails cut after they complete their first solo?
Supposedly, this is because in the early days of flight before intercoms were common instructors used to sit behind their students in a tandem aircraft and pull on their shirt tails to give directions. After successfully soloing, the student has shown that he doesn't need that direction any more and therefore doesn't need his shirt in one piece either.

This isn't a universal tradition: I had the end of my tie cut off to mark my first solo (in South Africa). I had to go out and buy a tie first!