Toes!

 Brian is quite into chewing on his toes.

 Sometimes it's hard to change his diaper because he will not let go of his foot.

It's like sucking your thumb but with your big toe!

 I thought it was a recent thing until I was looking back at the pictures that had piled up on our camera. This has been going on for at least a month. 

I guess toes are the best because they are always available.