"Powered by Condensation Nuclei"
Sure, it's a bit geeky - making a design about the particles that water condenses around to form clouds. It may confuse some people, but you'll get a good laugh from those who 'get it'.

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Condensation Nuclei Organic Cotton Tee Powered By Condensation Nuclei (you know, the particles around which water condenses and thereby forms clouds!) A bit geeky sure, but so what. (poweredbytshirts.com) $21.99 Size:  Size Chart Small Medium Large X-Large 2X-Large (+$3.00) Qty:    AVAILABILITY: Product Number: 29869941 Tell a friend about this product! Product Information Fit: Fitted Smaller cut, designed to fit tightly. Size up for a looser fit. Fabric Thickness: Lightweight for summer comfort or winter layering, our Men's Organic T-shirt is one of our most popular fine tees. It's what to wear when you care about having 100% organically-grown cotton next to your skin. The next best thing to nothing, naturally. 4.8 oz. Ultra fine combed ring spun organic cotton, great for layering Vintage fit (size up for a looser fit) Made in the U.S.A.