We're often told to "be here now." Yet the mind is rarely tethered in place. We take mental trips to our past, revisiting what happened yesterday or when we were children, or we project into an imagined future: tomorrow's dinner date, the trajectory of our career at age 50. Rather than a diversion from the norm of mindful presence, this tendency to internally visit other time lines, called "mental time travel," is common; young adults, for example, think about their future an average of 59 times a day. Psychologists have suggested that this ability to time travel from the confines of our own heads is a fundamental aspect of what it means to be human.