I wear black socks. Right now, I have two batches from two different brands. They look similar, but the thought of wearing two different ones makes me crazy, so I make sure to pair them correctly when folding my laundry.
It's hard to tell the difference between two of them, but I realized today, for the first time, that when you look at three together, it becomes very easy to tell the one that doesn't belong. I can basically do it at dusk with no lights on, using this method.
I feel like I've just made a personal discovery of one of the fundamental truths of logic.