A brilliant article from The Washington Post: That Dropped Doughnut (thanks, Emilee).
I remember hearing ten-second rule more than five-second rule as a child, but I could just have a faulty memory. I remember specifically, however, that my uncle Matt was the first person to ever explain the rule to me and I believed him because I was young and Uncle Matt is a doctor. I'm pretty sure I even used "Well my uncle's a doctor and he says..." as an excuse to my elementary school friends. This was before I found out that adults lie to children about what they know and know even less.