Car go, “Beep beep, vroom, vroom!”
Explanation: A knock knock joke to get you moving.
Cargo is the word for goods (things) carried by a ship, train, or truck from one place to another. A ship is loaded with cargo such as boxes, containers, cars, grain, … to move it from one place to another.
“Beep, beep, vroom, vroom!” are the sounds a small child might make while playing with toy cars. As the child makes those sounds, she or he might say in little-kid English, “Car go” meaning that the car makes noises like beep (the horn) and vroom (the engine) as it moves.
The joke is funny because in knock knock jokes you never know how it will end up. This one joins cargo with “car go.”
This is how ships are loaded with cargo: