The Crib: for sleep or play?

I went in to get Isaac at 2:00 this morning (yes, he still gets up in the middle of the night most nights, unfortunately) and he was sitting up in his crib! He's been able to sit for several months, but he's never sat up himself. I wonder if he cheated and got up on his knees and then fell onto his buns. It seems like he's figuring out so much recently. I feel like I'm going to walk into his room one morning soon and he will have climbed out of his crib, walked over to his dresser and gotten himself dressed. He's growing so fast! Here are some of the pictures of him on his knees.