What broke the windshield?

Isaac's head broke the windshield. Keith was moving one car out of the garage and putting the other one in, and Isaac rode with him in the passenger seat. Isaac asked for "drums", so Keith turned on the music. Isaac began to dance and his head hit the windshield.

His head was totally fine; not even a bump.

The windshield was not so fine, and we had to get the whole thing replaced.