Having grown up next to a park full of oak trees, I was well aware of the brown lumps you find under the leaves, but never knew what they were.  Never asked anyone either…  I now know, courtesy of the Chatelherault visitors’ centre, that they are made by the larvae of tiny gall wasps.
Courtesy of Wikipedia, I now know there are over 300 species of these wasps in Europe, and that the majority of these use only oak trees.  The gall provides a safe haven for the larvae, although other wasp species have found a way to parasitize these galls for their own larvae…  Nice…
I was stung by a wasp yesterday.  Bugger.  It’s been a while, though.
From Wikipedia