"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 Jr. Ringer T-Shirt 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 Color: Pink/Salmon Yellow/Gold Mint/Avocado Qty:    AVAILABILITY: Product Number: 29869940 Tell a friend about this product! Product Information Fit: Form Fitting Body-hugging close contoured fit. Size up for a looser fit. Fabric Thickness: Our women's ringer tees from Hyp are made of 100% fine cotton jersey. 4.8 oz. 100% fine jersey cotton Size up for a looser fit Contrasting neck and cuff trim as well as contrasting stitching