What grows on the oak tree?

usually, oak trees produce acorns but in some rare occasions, a certain species of wasp would lay an egg in the flower of an oak tree. when that happens, the flower develops into a weird-shaped "gourd" for the larva to feed on it. it drops in autumn and when spring arrives, the larva would have already developed into a wasp and would fly away. i am not lying. to prove it, go to YouTube and search for the video posted by BBC.