Well "gall"(y)

I've noticed these little balls on my oak tree over the years and finally decided to look into it.  There were never enough on the tree to suggest they might be early acorns.

Turns out, these "balls" or "galls" are protected a gall wasp.   They are also called "oak apples".

Oak apple galls form when a female gall wasp injects her egg into a budding oak leaf. The tree responds by forming a gall around the insect. The gall protects the growing insect and provides it with sustenance.

These are pictures I took over the past few days...

This is not my picture but shows one open...