What is on the under side of oak leafs?

While walking through the woods I noticed what looked like a marble stuck to the under side of most of the leafs on the trees.
They look like seed pods but they had been definately stuck there. They are red and orange in colour. What are they and what put them there?

Answers:
I think they are called oak galls. They are insect houses! Oak Wasps and other species make the "den" and hatch when they are ready.

Not saying definately but remember being told this on a nature walk when I was at school. ;)
It is an acorn that is infested with a larve
the oak gall wasp lives inside.
Red Mites is all I can think of just now.

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