One warm summer day, I heard the familiar tune of an ice cream truck outside my window. That night as I was going to bed I heard it again, faintly in the distance. But why was an ice cream truck cruising the neighborhood at nearly midnight? It wasn’t. Gradually I realized the jingle was not coming from outside my window. It had been playing inside my head.

Luckily, I’m not crazy. It’s true that some types of hallucinations are associated with mental illnesses. But many healthy people also experience “phantom” senses. Some hear music that isn’t there. Some see visions. Others live with imaginary smells filling up their noses.

Read More: Ghosts in your head | Student Science.