Facebook has been overhauling its Messenger app recently, slimming it down to focus more on chat, and promised that it would eventually roll out a dark mode for users.

That mode is now rolling out, but there’s a tongue-in-cheek trick you need to do in order to activate it: you have to moon someone.

The trick, spotted by Android Police, 9to5Mac, and others, is simple: send someone (or yourself) a crescent moon emoji ( ).

Once you do so, a shower of moons appears in the chat window, and you’ll get a prompt to activate the mode in settings. Go to your profile page in the app, and it’ll present you with an option to turn the mode on.

Android Police notes that the mode has apparently not rolled out to everyone and that some have gotten a message saying that it’s still a work in progress.