"In my youth," Father William replied to his son, "I feared it might injure the brain; But now that I'm perfectly sure I have none, Why, I do it again and again."
Why it works
This stanza maps precisely onto limerick form: lines 1, 3, and 5 share the A rhyme ("son / none / again"), lines 2 and 4 share the B rhyme ("brain / again"), the longer A-lines roll in a da-da-DUM anapestic lilt, and the fifth line delivers the classic limerick punchline—the absurdist admission that Father William has no brain—subverting the reasonable concern raised in the setup.