Goes up watermelon, comes down squash...
The city slicker in his sports car spotted a stand by the side of the road. All it had were watermelons. The guy asked the farmer what in the heck they were.
“Why - these here are dinosaur eggs.”
So the city slicker buys one and straps it onto the trunk rack of his sports car. Miles down the road he notices it is missing. He turns around to recover it but it is smashed on the pavement, and chickens from a nearby farm are pecking away at it.
The city slicker races back to the stand.
“I want my money back - that was no dinosaur egg! Nothing but a chicken egg!!”