It was the sound of Miles speeding through Milestones.
Apparently, sprouting two teeth more or less simultaneously is not enough for Miles. He wants to take over the world, so of course, he has to accomplish everything. Now.
Yeah, Miles has decided that it's time to sit up. All.by.himself. And boy, is he proud of himself.
After this picture, he sat up for another 20 minutes or so, before deciding that he had other things to do. He leaned himself back, one arm on the toy to brace himself, until he was lying on his back. He immediately rolled to his belly, and started trying to figure out how to scoot or crawl.
Apparently, he still has high ambitions. :)