Knobbly acorns

In our house we call them knobbly acorns, and at this time of year they are the bane of Rory's life as they get stuck in his feet whenever we go out for a walk across the village green.

Apparently they are actually called Knopper Galls. They are caused by a particular type of wasp laying eggs in the acorns. They mutate the acorns into green popcorn shapes, which go brown and woody and fall to the ground - and ultimately end up between Rory's toes. Bless him he does try to kick them out, but more often than not he needs a hand.