Clean As a Whistle

I heard this one today.  Now, I find it quite unlikely that this could apply to just any kind of whistle; mouth-blown whistles especially are anything but clean. My solution followes.

"Clean as a whistle" refers to a steam whistle.  In the past, steam trains travelled  from place to place, always announcing their presence by blowing a whistle.  The whistle was a powerful one, given that it's power came from the same steam that powered the engine.  This steam was under pressure, and it was boiling hot.  When it was blown through the mechanics of the whistle, it naturally sterilized the interior, therefore making it "clean as a whistle."  Quod erat demonstrandum.