Fish Joke (alright, I know they're mammals)

[Ignatzcatz]

What's the difference between a dolphin and a porpoise?
The dolphin didn't mean to do it.

Ok, that was a bit lightweight, we need something slightly lower down the scale. . .

Bill was a bit embarrassed and told the doctor he had trouble getting an erection with his wife and she was getting frustrated. The doc checked the man's blood pressure and other vitals, then after a thorough examination the doctor said he wanted to check Bill's wife.
He took her to another cubicle and asked her to get undressed. Then he told her to turn all the way around slowly. She did as instructed. He then told her to raise her arms above her head, then bend over, touch her toes and cough.
Finally he said, "OK, good. You can get dressed now and I will talk to your husband".
The doctor went back to the other cubicle and said to the husband, "Well Bill, you can relax, there is nothing wrong with you. I couldn't get an erection either."