Peter has learned how to roll from his back to his tummy, but seems to have skipped the step of learning to roll from tummy to back (my other two learned this one first). Today, I was practicing with him so he could start learning to sit up on his own. I didn't think he'd get it yet, because his siblings learned it when they were 6 months old, or a bit older. I let go of him to see if and how long he could balance on his own, and he had it for several seconds! He did this several times (with cheering on from his big brother).