"There," said the boy, and he was holding a tightly folded piece of paper. "Put this in your pocket," and she did.

"Now the way this game works," said the boy, "is that you give me a triplet of three numbers, and I'll tell you 'Yes' if the three numbers are an instance of the rule, and 'No' if they're not. I am Nature, the rule is one of my laws, and you are investigating me. You already know that 2-4-6 gets a 'Yes'. When you've performed all the further experimental tests you want - asked me as many triplets as you feel necessary - you stop and guess the rule, and then you can unfold the sheet of paper and see how you did. Do you understand the game?"

"Of course I do," said Hermione.

"Go."

"4-6-8" said Hermione.

"Yes," said the boy.

"10-12-14", said Hermione.

"Yes," said the boy.

Hermione tried to cast her mind a little further afield, since it seemed like she'd already done all the testing she needed, and yet it couldn't be that easy, could it?

"1-3-5."

"Yes."

"Minus 3, minus 1, plus 1."

"Yes."

Hermione couldn't think of anything else to do. "The rule is that the numbers have to increase by two each time."

"Now suppose I tell you," said the boy, "that this test is harder than it looks, and that only 20% of grownups get it right."

Hermione frowned. What had she missed? Then, suddenly, she thought of a test she still needed to do.

"2-5-8!" she said triumphantly.

"Yes."

"10-20-30!"

"Yes."

"The real answer is that the numbers have to go up by the same amount each time. It doesn't have to be 2."

"Very well," said the boy, "take the paper out and see how you did."

Hermione took the paper out of her pocket and unfolded it.

Three real numbers in increasing order, lowest to highest.