Enscribe

This cantrip spell lets the user write words of light in the air or on a surface. The words can remain there glowing forever (until dispelled) or programmed to only appear in certain events, such as someone entering the room, or saying the word “apple,” or chanting a spell, or when the moon is full, and so on. A favorite for mages out to inscribe code or other cryptic messages, especially if they are words they want hidden until the secret password is given. The words, visible or not, radiate a small magic aura, so they may be detectable. The real question is how to activate them. This spell is not taught in Magic Guilds, for the grafitti resulting would be annoying.