Courtesy of my father-in-law, aka the filthy joke repository.
An elderly man goes to his doctor complaining of discolouration in his semen. The doctor hands him a specimen bottle and tells him to bring back a semen sample as soon as possible.
Three weeks later, he comes back to the doctor without the sample. The doctor asks, "What happened?"
"Well," he said, "I tried using my left hand, then I tried using my right hand, and neither of those worked. Then I got my wife to try; she used her left hand, her right hand, using her mouth with her dentures in, without the dentures in, and she couldn't either. So then we went to the neighbor next door, he tried his left hand, then his right hand, then his mouth."
"And?"
"None of us could get the bottle open."
Three weeks later, he comes back to the doctor without the sample. The doctor asks, "What happened?"
"Well," he said, "I tried using my left hand, then I tried using my right hand, and neither of those worked. Then I got my wife to try; she used her left hand, her right hand, using her mouth with her dentures in, without the dentures in, and she couldn't either. So then we went to the neighbor next door, he tried his left hand, then his right hand, then his mouth."
"And?"
"None of us could get the bottle open."