A man’s driving down the road behind an 18-wheeler. At every red light, the truck-driver gets out of his cab, runs back and bangs on the truck door.
After seeing this at several sets of lights in a row, the car driver follows him until he pulls into a parking lot. When they’ve both come to a stop the truck driver once again jumps out, runs to the back and starts banging on the truck door.
The motorist goes up to him and says, "I don’t mean to be nosey but why do you keep banging on that door?"
To which the truck-driver replies, "Sorry mate, can't talk now, I have 20 tons of canaries and a 10 ton limit, so I have to keep half of them flying at all times."