Electronic devices capable of wireless communication are becoming ubiquitous. They enable a wide range of novel applications but these are often difficult to deploy in practice because wireless channels provide ample opportunities to attackers. A number of approaches have been proposed for building secure channels in these scenarios. An approach that would be simple, general and effective consists in establishing a shared secret between two devices by placing them in physical contact for a few seconds. This approach has not been exploited in practice due to the lack of common interfaces. We demonstrate the practical feasibility of the approach for devices equipped with a small LCD screen and cameras. We transfer secret keys between Android-based smartphones put in contact with each other for just a few seconds. The transfer occurs across a visual channel that cannot be intercepted.