Some of the children (and adults) were looking at these. – Spangle Galls – Some tiny wasps (cynipid wasp Neuroterus quercusbaccarum) lay their egg on the oak leaf, puncturing the leaf and activating it’s defences.
The leaf grows to protect itself and the egg encourages it, making the leaf produce a growth that surrounds the egg.
In Autumn the gall drops to the forest floor, then later the oak drops its leaves, covering the galls and keeping them warm and protected through the winter.
In Spring the eggs hatch, and feed on the gall itself which was provided by the trees leaves, but they only grow into a stage of the wasp, wingless like an ant. These then climb the tree and lay another egg which grows into another type of gall, which finally hatches into the wasp proper.
Amazing life cycle! More details here
We were watching a pigeon eating them on Monday, obviously they form an important part of the ecosystem.