This potentially lethal game is said to be played by fun-loving urban teenagers on warm summer evenings.
You soak a rag in petrol, roll it into a ball and wrap it in aluminium foil. Then, you stick a firecracker into the thing and walk up to a traffic light. When it turns red and someone stops with their window open, light the firecracker and throw the ball inside. Don't forget to shout "Spunkball!" as you do this. A splendid time is guaranteed for all.
Note that this is an urban legend and has not yet happened anywhere. Kids, don't try this at home.