I was performing a complete physical, including the visual acuity test.

I placed the patient twenty feet from the chart and began, "Cover your right eye with your hand."  He read the 20/20 line perfectly.

"Now your left." Again, a flawless read.

"Now both," I requested. There was silence. He couldn't even read the large E on the top line.

I turned and discovered that he had done exactly what I had asked; he was standing there with both his eyes covered. I was laughing too hard to finish the exam.