It's kind of a hard question to answer - basically I'm not really sure.
It might have been with my playmate when I was 5, upstairs in a big house in Palo Alto. Then again, as I found out at 17, my hymen was still very much intact - so much so that my boyfriend and I were still unable to do the deed even after I had moved in with him. Apparently tough hymens run in my family.

So then a few weeks (?) later there was this party we went to at a friend's apartment in San Francisco. We drank a lot and spent the night - the last thing I remember before I passed out is that we were going to try again...I'm not sure what happened after that. All I know is when we tried again the next day at home, it worked - no more ouchie. So maybe that's when it happened, in the house my boyfriend and I were living together in. I guess I'll never know.
