Face down, butt up!

Lately, when I put Zoe to sleep for a nap or for the night, she moves herself into a face down, butt up position.  Not sure why she does this, but it seems to be her favorite sleeping position at the moment. And she also prefers to be all the way into the corner of the crib when doing this.  As long as she can breathe properly, I guess it's ok, right? We'll see how long this one lasts! ...