A Stupid Calculator Trick by Michael Keller
I have no idea if this is a known trick; I found it by accident in 1992.
(1) Using a standard (non-scientific) calculator (e.g. the calculator
which is included in Microsoft Windows), calculate a reasonably
accurate approximation to pi using only two keys. You may hit
them as many times as you wish, in any order.
(2) Once you have done that, calculate an even more accurate approximation to e.
Hints (and link to solution)