A Stitch in Time Saves Nine

I realize that by this time most people probably already know the meaning of this phrase, but I thought I'd mention it anyway...

This is a reference to darning (repairing) socks.  The idea is that if you are bright enough to take care of the problem when it's a small one, you won't have to cope with it later when it becomes larger; i.e. one stitch repaired now will prevent having to repair nine stitches later.
Quod erat demonstrandum.

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.