Jon and Dave were in a mental institution. This place had an annual contest, picking 2 of the best patients and giving them two question.
If they got them correct, they're deemed cured and free to go.

Jon was called into the doctor's office first and asked if he understood that he'd be free if he answered the questions correctly.
The doctor said, Jon, what would happen if I poked out one of your eyes ?
Jon said, I'd be half blind.
That's correct. What would happen if I poked out both your eyes ?
I'd be completely blind. The doctor stood up, shook his hand, and told him he was free.

On Jon's way out,as the doctor filled out the paperwork, Jon mentioned the questions to Dan.
He told him what questions would be asked and the answers.

Dan was called in. The doctor went through the formalities and asked, what would happen if I cut off one of your ears ?
Dan, remembering what Jon had said, said, I'd be half blind.
The doctor looked a little puzzled, but went on, What would happen if I cut off both your ears ?
I'd be completely blind.
Dan, how can you explain that you'd be blind ?
My hat would fall over my eyes.