Mom replies, "No, because she is in heat."
"What's that mean?" asked the child.
"Go ask your father. I think he's in the garage."
The little girl goes to the garage and says, "Dad, may I take Princess for a walk around the block? I asked Mom, but she said the dog was in heat, and to come to you."
Dad said, "Bring Princess over here."
He took a rag, soaked it with petrol, and scrubbed the dog's backside with it to disguise the scent, and said "OK, you can go now, but keep Princess on the leash and only go one time round the block."
The little girl left and returned a few minutes later with no dog on the leash. Surprised, Dad asked, "Where's Princess?"
The little girl said, "She ran out of petrol about halfway down the block, so another dog is pushing her home."