There are 3 boxes. One contains only Apples. One contains only ORANGES. One contains both apples and oranges.

All the 3 boxes are incorrectly LABELED. The labels are Apples, Oranges and 'Apples plus Oanges'

You are allowed to open just one box. Without looking inside the box, you can take out one piece of the fruit inside. By looking at the fruit, you have to correctly label all the 3 boxes. That is identify which box contains apples, which box contains oranges and which one contains both.