Opportunity cost in a very literal way.

In a large studio stands a giant perspex room with a single, sliding security door. Outside the door there is a computer terminal, blinking a cursor as it invites you to enter the answer to the philosophical riddle it displays. Cramming knowledge or arranging cough codes with members of the audience won’t help you here - the only way to get the answer right and open the door is to think it through. You have as long as you like - under the studio lights and in discussion with the host.

Inside the perspex room is a grid of 400 shredders, twenty rows of twenty. Each has a device mounted over the inlet that, once the shredder is turned on, will feed £10 notes into the blades, one by one.

The game is simple - sit at the terminal, try to solve the riddle and input the correct answer to open the door. However much money is in there when you do is yours. But if you get the question wrong, the computer will use a number of factors about your incorrect answer, plus a bit of randomness, to decide how many shredders to turn on and how long for.

You could win a million pounds, or you could destroy a million pounds. Your choice.