"It came in when I was about six," she recalls. "I had a younger sister who ran into things a lot, and I worried about her, and then one day I could just tell she was okay. And the next time she got overexcited and had an unfortunate encounter with a door, I could tell she wasn't hurt. As for why it happens, your guess is probably only slightly worse than mine. They're rare enough that it's hard to see any kind of pattern."