My first roll-over

I know this might be hard to believe, but I have to tell you about my first roll-over. In fact, it happened so unexpectedly that I was very surprised myself too. It happened like this: I was lying on bed with my parents at around 11 pm tonight. They were trying to get some rest before my next bout of crying, and I was beginning to warm-up for my full-force crying. Then I realized that if I use one of my legs to push on the mattress and twist my waist, my hips will be in a more comfortable position. A few pushes with that leg, and voila, I had my first official roll-over, unassisted and perfectly legitimate. Now can any one tell me when I was supposed to do this, normally?
Oh, you should have seen the look on my parents faces!!