Deaf people commonly use lights to communicate with one another in lieu of voices that we cannot hear (the other option is to use our feet to stomp on the floor or our fist to pound on the wall to create a loud, low-frequency vibration that can be felt).
Anyway, Mike and I are no exception. Luckily, we designed our house so that you can turn the upstairs lights on/off when you are downstairs and the downstairs lights on/off when upstairs. Thus if I am downstairs and need to let Mike know that dinner is ready, I will stand at the bottom of the stairs and flick the upstairs lights.
I was upstairs doing laundry a few days ago while Jake was downstairs watching a movie. All of a sudden the lights flicked. There was Jake, standing at the bottom of the stairs, waiting to talk with me!! Amazing.