John has been able to roll back-to-tummy for a few months; I know this because I saw him do it a few months ago, although not exactly on purpose, but nevertheless the ability was there. However, he hardly ever did it, preferring to avoid being on his tummy as much as possible. Until a few days ago. He seems to have suddenly realized that rolling can help him get places.
I've also noticed that when he pulls up to stand, he'll pull about halfway up with his arms, and then grab on with his mouth as well. Why, I have no idea.