A couple went to see the obstetrician, because their young daughter had red hair.
“Doctor”, the girl’s father said, “I don’t mind telling you, but I am really concerned about our daughter. She can’t possibly be mine because she has red hair”.
“That’s nonsense”, the obstetrician said. “Even though both you and your wife have straight black hair, one of your ancestors may have contributed red hair to the gene pool”.
“It just isn’t possible”, the man insisted. “This just cannot be, our families on both sides have had straight jet black hair for generations”.
“Well”, the doctor said, having thought for a minute. “Let me ask you this. How often do you have sex?”
The man seemed a bit ashamed and bowed his head. “I’ve been working very hard for the past year, we only made love once or twice every few months”, he said.
“Well, there you have it”, the doctor said confidently, “it’s rust!”
Image used under a Collective Commons License from https://pixabay.com/photos/red-hair-red-head-little-girl-6071694/