Here's a short and easy one:
Me and my friend were hanging out in a restaurant and on the way back we saw a strange looking, abandoned castle. We wanted to check it out, so that's what we did.
The lights were off, and we saw 3 switches, and it wasn't immediately obvious what they did. For example, when one of those switches was turned on, it turned off the lights although the other 2 switches were on.
I told my friend it was weird that each switch didn't just turn on a light and asked if he knew how these lights worked, and he said: