(CNN)Billie Eilish visited "The Tonight Show" on Monday, where she performed her song "Happier Than Ever" off of her new album of the same name.
Eilish sang the song on a large stage backed by her brother Finneas, who is also her producer.
She also chatted with Fallon about how her new album came about.
"It really just happened. I, like, didn't know what I was going to do at all until really almost when the album was over. I didn't even know what it was going to be called, I didn't know what the vibe was going to be, I didn't know what the artwork should be. I knew, though, that I wanted it to feel very specific, you know? I wanted it to have a real... feeling to it and a real aesthetic to it."
She also discussed her synesthesia, a brain condition in which a person associates colors and numbers to people and objects. For instance, she sees Tuesday as purple, while her father, who also has synesthesia, sees it as orange.
As for Fallon, Eilish says he is a "vertical brown rectangle," and the letter J in Jimmy is also brown according to her.
You can check out their conversation here.