Are we on our way?

We thought Henry was a long way from crawling because as soon as you put him on his stomach, he rolls to his back. Very gracefully, but nonetheless, he ends up on his back pretty quickly. So we figured he wouldnt be able to crawl if he never spent time on his stomach. Well, now Im not so sure. He was ever so tired after our fun packed picnic at the park with Papa at lunchtime (say that quickly!) that I was sure he would go straight to sleep when we got home. So I put him on his back, in his bed and sure enough, he was quiet straight away. Then 3 minutes later he was screaming bloody murder. Henry isnt one to cry when you put him to bed unless something is wrong, and it usually means wind, so I went to see what was up. My crazy little boy was at the other end of the bed, 90 degrees to the way I had positioned him and on his front. Not lying on his tummy but propped up on his elbows and knees, just a-cryin' his little heart out with dark purple tired eyes. So I picked him up and asked him why he didnt just roll onto his back if he didnt want to be on his tummy any more, but of course, he couldnt answer me. I had read somewhere that babies are thinking about all the things that they are learning so much that they think about it in their sleep too. So I wonder if he was thinking about crawling in his sleep and rolled over in his sleep and thats what woke him up. We shall never know, but I wonder how far away mobility really is...