In season 1 of "Stranger Things," Joyce Byers communicates with Will using Christmas lights.
After Will disappears into the Upside Down, Joyce refuses to accept that he is dead. Strange electrical disturbances around her house convince her that Will may be trying to reach her from another dimension. She strings Christmas lights across the living-room wall and paints an alphabet beneath them, assigning one letter to each bulb.
Will cannot see the alphabet itself, but he can affect the corresponding lights from the Upside Down. Joyce asks questions aloud, and Will answers by making individual bulbs flicker. The scene turns an ordinary holiday decoration into an improvised communication device and becomes one of the season's most recognizable images.
A common mix-up is the telephone: Will also makes contact through phones and electrical signals, but the alphabet-and-lights exchange is the specific method described here. The Christmas lights do not let Joyce see the Upside Down directly; they provide a way for Will to signal across dimensions.