How can switches across the room from each other control the same light in my house? Are they in the same circuit? Is there a limit to the number of switches?
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Say there’s a walking path that passes through a tunnel that carves through a hill. The tunnel has two separate lanes that run parallel to one another, but aren’t connected. At each end of the tunnel is a sliding door that always blocks one of the tunnel entrances, but not the other.
Say you start out with one of the lanes completely shut, at both ends. Since each door can only block one lane at a time, and both of them are currently blocking the same lane, it means the other lane must be open. People can walk all the way through, so traffic flows happily through the tunnel.
Now, slide one of the doors over. You now have opened up the previously closed channel at one end, but at the same time, shut the channel that used to be open. Now no one can get through either channel, because both of them are blocked on at least one side. Traffic flow through the tunnel comes to a screeching halt. This will happen no matter which door you choose to move.
Pick another door and slide it. One of two things will happen: either you pick the same door you moved the first time and move it back to where it was when we began, or you pick the other door, and move it over to the same lane you moved the previous door to. Both situations get you the same result: you now have one fully blocked channel and one fully open channel, meaning traffic can flow once more.
With this system, you can always toggle the tunnel between open and closed just by sliding one of the doors. And it never matters which door you slide–all that matters is every time you slide a door, the tunnel toggles to the opposite state.
If you substitute tunnels and traffic flow for wires and electricity flow, and swap the doors for switches, you have the exact mechanism that lets you control a light with two switches.
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