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) Calculate an even more accurate approximation to e, again using only two keys.

Hints (and link to solution)