Suppose you were in two-way contact with some alien species, but only by "telegraph" (i.e., light flashes or radio signals). The well known procedures of SETI, starting with prime numbers and progressing to pictures, physics, and chemistry information could be used to develop a common language and arrive at a good level of communication. You could tell the alien how tall you are by expressing your height in mutually understood wavelengths of light. You could tell the alien how old you are as some large number of ticks of a light-frequency clock. Now you want to explain how humans shake hands when they meet, and you describe extending your right hand. "Wait a moment!" says the alien. "What do you mean by ‘right’?"
http://www.lbl.gov/abc/wallchart/chapters/05/2.html
http://en.wikipedia.org/wiki/CP_violation
