It is a game based purely on chance, but remained suspenseful because both rewards and risk increased as it progressed.
The game was invented by David Briggs, a former traffic reporter for Capital FM who went on to create Who Wants to Be a Millionaire?
[1] The contestants were primed by Capital Radio's Breakfast Show host, Chris Tarrant, whose collaboration with Briggs led to him becoming the presenter of Who Wants to Be a Millionaire?
Players shouting "stop" before the "bong" wins the last sum of money mentioned.
An additional variation was the "Bouncing Bong" which, instead of just going up, went up and down randomly, with the contestant having no idea whether the next number would be higher or lower than the current one.