"Remember those British schoolchildren who made up answers about Mary's trip to the seashore? The researchers who ran that experiment did a follow-up study, called "Helping Children Correctly Say 'I Don't Know' to Unanswerable Questions." Once again, the children were asked a series of questions; but in this case, they were explicitly told to say "I don't know" if a question was unanswerable. The happy news is that the children were wildly successful at saying "I don't know" when appropriate, while still getting the other questions right."