Once, having placed her in the middle of the rug, I came back in the living room and couldn't find her for a minute. Leah was under the couch, quiet as can be.
I shouldn't have been surprised, since this happened when she was not quite 3 months old!
Here is a picture of her trying really, really hard at 5 months:
And still not quite there, but so close at 5.5 months. It's been very exciting at our house for the big kids.
I wanted to get a little better video, since she really can go a long way (from the living room all the way through the kitchen to the dining room, for example) and she gets up on all fours to do it, but after several attempts, this is the best effort I have on camera.
6 months old:
Now she's crawling fast and cruising along the furniture. James very, very, very excitedly reported that she let go and stood for a second (this was independently confirmed later).
Leah at 9 months old: