You've probably been in this trap: you enter an unfamiliar restaurant, pick up someone else's conversation out of your ear, and suddenly feel cold because it seems you've been through this situation before. Or you meet a person, then painfully try to remember where you might have seen their face.

You've probably been in this trap: you enter an unfamiliar restaurant, pick up someone else's conversation out of your ear, and suddenly feel cold because it seems you've been through this situation before. Or you meet a person, then painfully try to remember where you might have seen their face.