I remember hearing of a puzzle some time ago that went along the lines:
There is a humanoid alien race on a far distant planet that has never encountered the human race.
Purely by spoken instruction (i.e. no visual communication, and you cannot make use of scientific attributes such as polarisation), you need to inform this race how to identify left and right.
Since they are humanoid you can assume that they share the concepts of up/down front/rear, and left/right - they just do not know which name for the latter describes which side. You can also assume they can do the same everyday functions that we can.
Now, it was stated that this is impossible, but I think I’ve missed something as it seems a fairly simple thing to do.
Does anyone know the full rules to the problem that make it impossible?