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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:

______, duh!

Is the answer to this

Series-ly?

Because

The lights and switches work in series (as opposed to in parallel)?

Electricity, duh!
You asked how the lights worked, not the switches. Your friend responded in kind.

Partial answer (?):

There's a lot of weird stuff on this one, and I don't know if it's intended to guide us towards the solution, but abandoned castles shouldn't have electricity in the first place, and if the lights were off, you probably couldn't see the switches either. I'm maybe guessing this is a toy castle rather than a full sized-one, but this is just a very incomplete attempt at an answer.

Alright, how about:

a Code or Combination?

explanation:

The kind of lights aren't explained. Maybe you need all the lights to be green to get into the castle or something.

Probably isn't. Last attempt for now, don't want to spam.

Is it one of these?

Nope, duh!

or

Magic, duh!

or

Exclusively, duh!

or

Multiway switching, duh!

Lights-out, duh!

The way that the switches work reminds me of

Lights Out, a game where pressing a button reverses the state of the buttons around it. It is not intuitive to turn all of the lights on or off.