An Engineer, a Chemist and a Standards Designer...

An engineer, a chemist, and a standards designer are stranded on a
desert island with absolutely nothing on it.  One of them finds a can
of spam washed up by the waves.

The engineer says "Taking the strength of the seams into account, we
can calculate that bashing it against a rock with a given force will
open it up without destroying the contents".

The chemist says "Taking the type of metal the can is made of into
account, we can calculate that further immersion in salt water will
corrode it enough to allow it to be easily opened after a day".

The standards designer gives the other two a condescending look, gazes
into the middle distance, and begins "Assuming we have an electric can
opener...".
                             - from Peter Gutman's X.509 Style Guide