"Question for Mother" joke

Muriel was in a serious mood and asked her mother for advice. She was about to be married and the groom had come to stay with the family for the weekend during which time she had blundered into the bathroom and saw him naked.

"Mother," she said, "what was that thing hanging down between his legs?"

"They call it a penis my dear, nothing to worry about."

"And the knob on the end, like a firemans helmet."

"Just the glans," said Mother.

"And there are two round things about 13 inches back from his glans. What are they?"

"For your sake, my dear," said Mom, "I hope they are the cheeks of his arse!"