a bob of the head的用法和样例：
- She greeted us with a nod of the head.
- There is a bolster across the head of the bed.
- I got him on the back of the head with a crowbar.
- The end of the head on a golf club.
- I get him back of the head a crowbar.
- Father gave a bob of his head when I asked if I could go out.