An elderly couple goes to the doctor, both complaining of memory loss. The doctor checks them out and tells them, "Well physically, you're fine. But at your age a certain amount of cognitive decline can be expected. Unfortunately there's nothing we can do about it. What I suggest you do is just carry around a pen and a pad of paper, and if there's something you need to remember just jot it down." So they do, and it works well.
A couple of weeks later they're sitting around watching TV, and during a commercial the husband gets up and walks towards the kitchen.
"Where are you going?" asks the wife.
"I'm just getting a glass of water" he says.
"Would you make me a peanut butter and jelly sandwich while you're there?" she says.
"Sure" he answers, and continues walking.
"Aren't you gonna write it down?"
"Nah" he says. "I can remember peanut butter and jelly from here to the kitchen."
Two minutes later he comes back and hands his wife a tuna fish sandwich. She looks at it in disbelief. "You idiot" she says. "I knew you should have written it down. I said no mayo!"