Sitting up

Today I found Jonathan sitting in the crib looking around when I went to pick him up. Somehow he had figured out how to sit up by himself. He hasn't done it on the floor yet. Maybe the crib bar helped. He managed to drink some water out of a sippy cup once. Then he couldn't anymore. I don't know how babies figure out how to use this thing. I tried it once and it took quite a bit of force the get the water out. Maybe I didn't get the right brand. He is crawling really fast now, not on all fours but he has learned to alternate the arms with the belly crawl. His favorite places to go toward are: the kitchen, the front door, and the closet. His favorite game is throwing a small plastic ball against the floor, see where it falls and then go get it. Then he will pick it up, throw it again and go after it. He can entertain himself a good while with this game. We just put up the gates now so we can let him roam at will. We also laid down foam play mat. He loves the bright colors. I still worry about him falling backwards from a sitting position but at least the impact is less. His two front incisors have been barely visible in the gum for a few days now. When will they come out?