Joke of the day - take II

Mrs. Biddle was walking down the street one day carrying a small box with holes punched in the top.
“What’s in that box?” Mrs. Riddle asked.
“A cat,” Mrs. Biddle answered.
“What for?”
“I’ve been dreaming about mice at night, and I’m scared of mice. The cat is to catch them.”
“But the mice you dream about are imaginary,” said Mrs. Riddle.
Mrs. Biddle turned to her friend and whispered, “So is the cat.”