It was my sister Alice's birthday, and I had a present for her. As you know, Alice is a very intelligent girl, and as you also know I like presenting little problems. So I gave Alice three boxes, one wrapped in red, one in yellow and one in blue. In one box, I told her, is your birthday present; the other two are empty. "Now," I said, "you have to determine in which box is the present. You can ask me any number of questions answerable by yes or no, and I'll either answer them all truthfully, or else I'll lie each time." First, she asked if the present was in the yellow box. I answered her, yes or no. Then she asked if it was in the red box, and again I answered yes or no. And Alice, after some thought, knew which box contained the present. How?