You have three boxes in front of you, one with nails in it, one with screws, and one with both nails and screws. The boxes all have labels describing their contents. Unfortunately, all labels are incorrect, and describes one of the other boxes.
You can pick up one item from one of the boxes (no peeking). How do you determine what is in each box? Put another way: From which box, with which label, do you take an item?