I'd say the #2 redwood pencil. Or pencil & paper considered as a system.
>The three-legged stool?
Not bad, and "Tripos" has a rich historical association--the final exams
in England where an upperclassman would sit on a 3-legged stool (hence
the name) and improvise insults and challenges.
> The knife? The funnel?
How about the Inclined Plane?
> Lee Daniel Crocker <lee@piclab.com> <http://www.piclab.com/lcrocker.html>