John and Chris played trumpets. They always stood back to back while playing, as they believed the sound filled out the room better that way. Since eye contact was not possible, they had to develop another way of communication. They created a body code for each message they wanted to signal. They knew each other so well that no eye contact was needed during the performance. They married twins on the same day and they used to spend most of their time together. They even rented a condominium together. Everything worked excellent except for one day when John saw something that Chris, who was facing the opposite direction could not see.