[hexane360]

I just point them towards Unlambda. Turing completeness in three characters (plus one for output)

http://www.madore.org/~david/programs/unlambda/#what_is

[lokedhs]

Unlambda as a language may be a joke (the funny kind), but learning it is incredibly enlightening, and makes you understand some fundamental concepts of computer science.

As such, I think it represents one of the most enlightening esoteric languages out there together with Brainfuck.