Answer:
I believe the pair of lines that expresses a humorous tone is:
But, now that I'm perfectly sure I have none,
Why I do it again and again."
Step-by-step explanation:
The pair of lines above is dictating the humorous tone of the poem. A father, someone who is obviously older than the son, and who should be expected to be wiser, claims to have no brain. Of course, not having a brain is impossible. He might mean that he has lost his mind, which might very well explain why now, being an old man, he chooses to stand on his head. That was something he should have done when he was younger, stronger, and less wise. It seems that this father is doing things backwards. His body is growing older, but his mind is growing younger.