Last night at 3 am DS woke us with his wailing. He had pulled himself up to a sitting position, but didn't know how to lay down and get back to sleep - he was pretty freaked out! As soon as I picked him up, though, he got an ear-to-ear grin as if to say, "Did you see my new trick?"
He also demonstrated it for DH while waking up this morning. I'm assuming the middle of the night wailing will stop in a few days. He freaked out the first few nights he could roll over, too.