One of the most baffling superstitions I've ever heard while living in Japan came from my mother-in-law. One day we were walking on a trail with my kindergarten-aged son when we looked down to see there was an earthworm crossing our path. We stopped, but before I could find a stick to nudge him out of the way, my mother-in-law screamed, grabbed my son, and yelled, "Don't pee on it!". I didn't know where to start. I think I started by explaining that her grandson doesn't usually make a habit out of dropping trou and piddling on every bug he happens to come across. But, also, was the looming question, why? I mean aside from the fact it's not a cool thing to do to such a tiny creature. Why? So I asked. She went on to lecture me about how little boys like peeing on worms (It's what she said, really.) and how if they do, their little boy parts will swell up and start itching terribly. It's an awful thing, she told me. Oookay. Keep in mind, this was pre-Internet, so there was no way for me to whip out (heh) my phone and check. I decided it was probably a silly old wives' tale made to keep rambunctious little boys from doing mischievous things. I even heard it a couple times after that fateful day, from different people. But still there was no insight into why this idea even started in the first place. Then one day, many years later, I was watching a Japanese TV show doing a bit about superstitions and this one came up. Some research was done and it turns out there is a kind of worm that when frightened expels some low grade poison that can travel up a urine stream to reach the offending genitalia. Swelling and itching ensue. I've still never met anyone willing to admit this has happened to them, however. Not that I don't ask now and then.
