Yesterday, my son and I found a spiny, green, spherical seed pod (approx. 1" in diameter) on the side of the road. Here's photos of the pod (note the small stem) and the pod when opened (interior was silky white in nature, with two pignola-like seeds). The spines are quite prickly--not soft at all, which is what I had been expecting considering how many there were and how small they were. The neighbor kid also pointed out a larger, already open seed pod (approx. 2-3"?) that he had found the previous evening. These are the only examples we've found, and I've never seen them in the neighborhood before.
We (I) checked online for candidates--I was thinking it might be sweet gum or sycamore, but the spines on our pod aren't spaced like the examples I found online. We are located in SE Penna. (USA); our house is surrounded by mature tulip trees, ashes, sweet birches, maples, dogwoods, and oaks.
The kids and I are excitedly awaiting any suggestions! Thanks in advance!