Baptiste wants to catch a pair of socks in the dark. He has 6 red socks, 13 blue ones, and 8 white ones. How many does he need to take to be sure of having 2 of the same color?
Here, the drawers are the colors, because we want at least two of the same color. The socks are the socks. There are three colors, so we take four socks.

We observe that the drawers are a "property": the color. Each sock must have one of the 3 colors, so we can categorize them accordingly.