To clarify, since several people asked... It was not during any sort of attempt at a strange Kama Sutra pose... It was just one of those 'see if you can do it' sort of things when I was about 9 or 10.
I was long-legged, skinny, flat chested and limber and I'm sure one of my sibs probably issued the challenge to see if I could put both feet behind my head, because I showed them I could put one foot behind my head. It was probably one of those snide comments like, "So what... But can you get both of 'em up there?" Naturally, I had to do it just so I could say, "Naner, naner, poo, poo!" at them because I could do it.
I will say, since we're keeping it real, that when I got older there were times when I dang sure could've put them behind my head if I'd thought about it, since they were back to my shoulders anyhow...