After their baby was born, the panicked father went to see the obstetrician. Doctor, the man said,

I dont mind telling you, but Im a little upset because my daughter has red hair. She cant possibly be mine!

Nonsense, the doctor said, Even though you and your wife both have black hair, one of your ancestors may have contributed red hair to the gene pool.

It isnt possible, the man insisted.

This cant be, our families on both sides had jet-black hair for generations

Well, said the doctor, Let me ask you this: how often do you have sex?

The man seemed a bit ashamed, Ive been working very hard for the past year. We only made love once or twice every few months.

Well, there you have it!, the doctor said confidently.

Its rust!

Aussies Paul.