When we first see someone we're attracted to, our eyebrows rise and fall. If they like us back, they raise their eyebrows. The whole thing lasts about a fifth of a second and it happens everywhere in the world -- to everyone regardless of age, race, or class. Lifting our brows pulls the eyes open and allows more light to reflect off the surface, making them look bright, large and inviting. A flash might be easy to miss but they're so reliable, if you do spot one, you may know someone likes you before they've even registered it themselves. Deliberately extend it for up to one second and you've drastically upped the chances of him getting the message you're interested.