I'm still scratching my head over "as keen as a whistle", noted above. What's keen about a whistle?

The sound! From my trusty, never dusty Webster's:
keen adj. 1. Very sharp, as a knife. 2. Cutting; piercing, as wit. 3. Vivid; pungent. 4. Having or exhibiting sharpness or penetration. 5. Acute: keen sight. 6. Exceptionally intelligent. 7. Characterized by intensity; a keen appetite. 8. Informal impatient; eager: keen to be off.

Based upon these definitions, I would clearly say that keen as a whistle refers to the sound a whistle makes. [plugging my ears-e]