Jon and Dave were in a mental institution. This place had an annual contest, picking two of the best patients and giving them two questions. 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 if I poked out both of your eyes?"  "I'd be completely blind." The doctor got 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 exam to Dave. He told him what questions were going to be asked, and told him the answers. Dave was called in. The doctor went through  the formalities and asked, "What would happen if I cut off one of your ears?" 

Dave, remembering what Jon said was the correct answer, he said, "I'd be half blind."  The doctor looked a little puzzled, but went on. "What if I cut off both of your ears?"  "I'd be completely blind." Dave answered.  "Dave, can you explain how you'd be blind?"  "My hat would fall over my eyes."