Some similes make sense.  Many don't.  Similes are used for comparisons and attempt to emphasize the comparison.  Consider "light as a feather".  Depending on the feather, if you want emphasis you could find things lighter than a feather like a crumb, a ladybug, or the obvious, air.  How about "clean as a whistle".  Some whistles are not clean at all.  In fact, they can be pretty messy.  Maybe it's the clean sound because whistles have few overtones.  Consider "fast as the wind".  Too bad the simile predates 1970 when U.S. National Hurricane Center developed the Hurricane scale with Categories 1 through 5.  It could have been "fast as Cat 5".

Once a phrase catches on, similes are forever like "sharp as a tack".  Who uses tacks anymore and what's the meaning of sharp?  Read Sharp as a Tack.

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