I came upon this very clever but simple proof recently ...
It is not known which
irrational numbers x and
y would make
xy rational. But it can be easily proved that such rational
xy exists. We know that
√2 is irrational. Take, for instance, the number
which must either be rational or irrational. If it is rational, then we're done. If it is not, then the number
must be rational. So either case proves the existence of rational
xy where both
x and
y are irrational.