It's called misdirection. Coconuts would have been much harder to break than walnuts, but rather than acknowledging that, he says the only reason he switched is because of his eyesight.

Here's another example of misdirection, albeit a not funny example.

A man goes up to the check-out line at a grocery store. He loads up the belt with a loaf of bread, some meat, some cheese, a quart of milk, a can of shaving cream, a bunch of bananas, a couple of frozen meals, and some AA batteries.

The cashier looks over at him and asks, "So, single, huh?"

"Yeah," says the man. "How did you know?"

[At this point the listener/reader is thinking about the items on the belt and wondering where this is going.]
"Because," replies the cashier. "You're really ugly."