I call them "Death Balls".

[Tom@WashDC]

They are hard and firm like wood, you cannot crush them with your hands or tires. When I hit a patch of them, it's like riding on ball bearings! They are around every tree and on the hills where I ride my fat bike right now. Quite treacherous little buggers. (sunglasses for scale).

 

[Ebiker53]

Sweet Gum seeds.

 

[rich c]

Sweet Gum seeds.

Actually seed pods, the seeds are inside (if they are still in there). They squish real easily if they are wet.

 

[ElevenAD]

I remember those as a kid, they were a pain in the butt!