There are two states of the world, full F and hungry H. The state is known to the agent/child but the principal/parent does not know the state. The agent has private information. How does the principal work out the true state? Offer the agent another french fry. If it is accepted, the true state is H – he is truly hungry and only pretending to be full. If he refuses, it is F and the chips he had earlier filled him up. Of course you have to know your kids to determine which food product separates or screens the two states.