Three closed boxes hold fruit: one holds only apples, one holds only oranges, and one holds a mixture of the two. Each box carries a label — 'Apples', 'Oranges', 'Mixed' — and it is known that every single label is on the wrong box. You may draw one fruit, without looking inside, from exactly one box, and must then label all three boxes correctly. Which box should you draw from?

Because its label is wrong, the box marked 'Mixed' cannot hold the mixture — it holds one kind only, so the single fruit you draw names its whole contents. Suppose it is an apple: that box is the apple box, the box marked 'Apples' cannot hold apples and cannot be the mixture (already found), so it holds oranges, and the last box is the mixture. The trap is drawing from the box marked 'Apples': if an orange comes out, that box could be the orange box or the mixed box, and you are left guessing.