An elderly man and an elderly woman are in a nursing home. Both are widowed and are looking for some companionship, and for the old man, perhaps a little more. Every night after dinner is over, the old man wheels himself down to the old lady's room and pulls up next to her chair. There, they sit side by side and watch television together. While sitting there, the old lady softly grasps the old man's penis and holds it in her warm hand all evening. Unfortunately, it does nothing for him.
Night after night, around the same time, the old man and old woman get together and do the same routine, but his penis remains flacid. The old man grows more bored with the setup and more frustrated with the progress he sees in his reawakened sex life.
Finally, one night after dinner, the old lady waits for the old man to come down to her room, but he never shows up. She gets a little worried thinking maybe he is ill or had an accident in his wheelchair. The old lady goes down the hall to look for him. A few rooms down from hers, she finds the old man with another old lady. They're sitting side by side watching television, and the other old lady is gently grasping his penis with her hand, but now he has a raging erection.
Dumbfounded and hurt, the old lady storms back to her room where she sits the rest of the night all alone and in tears. The next day, the old man wheels down to her room and asks her what made her so angry the previous evening. She responds by asking him a question. "Tell me, what does she have that I don't have?"
The man answers resoundingly. "Parkinson's!"