One of the most memorable scenes from Stranger Things S4's main villain, Vecna.

Season 1 was all about the disappearance of Will Byers, the dangerous experiments taking place at Hawkins Lab, and Eleven’s escape from it after accidentally opening a gate to another dimension. Will is the only person known to have survived various days in the Upside Down, but what makes his experience even more interesting and intriguing is that he was able to communicate with his mom by manipulating the lights in his house, even more after Joyce set up Christmas lights all over the house and over an alphabet painted on the wall.

Related: Stranger Things: How Will Survived The Upside Down

The Stranger Things Christmas lights were Joyce's big break in figuring out how to communicate with her son, followed by Eleven ing Will through the walkie-talkie by using her psychic capabilities. Much of the Upside Down remained a mystery for the first 3 seasons. However, Stranger Things season 4's ending shed some light on the mysterious dimension, including how the gates form, what the Upside Down actually is, what Vecna's ultimate plan is for Hawkins, and how the lights worked for Will and Joyce in season 1. Here's how Will was able to communicate with his mother from the Upside Down.

3 Seasons Later, Stranger Things' Christmas Lights Mystery Was Cleared Up

Joyce puts Christmas lights in her house to communicate with Will

In season 4, episode 7, "The Massacre at Hawkins Lab," a huge part of the Upside Down was explained, along with how time and energy work there. Nancy, Steve, Robin, and Eddie travel to the alternate dimension through a gate in Lover's Lake (so fondly referred to as "Watergate" by Dustin). As they make their way through the Upside Down, they visit the Wheeler house and realize that the dimension is actually a version of Hawkins stuck in 1983 – precisely the day that Eleven created the first gate to the Upside Down when she banished Henry Creel/Vecna there with her psychic powers. The teens then hear Dustin talking with Lucas and Erica about how and why Vecna is creating new gates all over town. They see strange illuminations around the light fixtures and use their hands to make the lights flicker, getting Dustin and the gang's attention with the Stranger Things Christmas lights trick. After this, Dustin and his friends are able to communicate with the teens in the Upside Down via a Lite-Brite.

The scene not only explains why and how Stranger Things electricity effects play a key role in the latest season, as the team also tracked Vecna through the lights. While there is still plenty of mystery surrounding the Upside Down, at least the question of the lights has finally been answered.