If you have 5 minutes to waste, and you aim at expanding your “poo-bililites” play the “Whose Poo” advergame on the Denver Zoo website.
Sorry, not direct link, you’ll find a small banner on the right side of the homepage. Yes, it’s kind of hidden in the site… and yes, you can wonder why 
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Here’s the “direct” link : http://denverzoo.org/includes/pooContainer.html