There was a spinster

There was a spinster schoolteacher who was plain, skinny and with an impediment in her speech

She longed for a man and used to comfort herself by reading respectable erotic stories, namely the 'Adventures of the Great Classical Gods'

This only made her worse, but one night a strange thing happened - she dreamed that a big blonde naked man came to her bed, ripped off her nightgown, and raped her again and again.

In the morning as he was about to depart, she begged him to stay. "I must return to Valhalla," he said, "I'm Thor."
"Tho am I, but wathent it marvelouth."