species identification - What sort of seed pod is this?

My son found this floating in a stream in Massachusetts, in the northeastern US. It was intact when he found it, but he cracked it open to see what was inside. I did some Google image searching, but couldn't find anything. Any idea what it is, and where it came from?

Answer

It's actually not a seed pod, but rather a gall (see here or here for more info about galls). Specifically, it most likely is a gall belonging to the Oak Apple Gall Wasp (Amphibolips confluenta). You can read about this species and its gall here.