If 90% of everything is crap, then we can separate everything into ...
crap 90%
non-crap 10%

Now if 90% of crap is crap (and hence 90% of non-crap is actually crap), then ...
crap crap 81%
non-crap crap 9%
crap non-crap 9%
non-crap non-crap 1%


As crap crap is a double negative, we now have...
crap 18%
non-crap 82%

Thus proving that "90% of everything is crap" is part of that 90%, or is that 18%....