"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 Ash Grey 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) $19.99 Size:  Size Chart Small Medium Large X-Large 2X-Large (+$3.00) 3X-Large (+$3.00) Color: Natural Ash Grey Light Blue Qty:    AVAILABILITY: Product Number: 29869945 Tell a friend about this product! Product Information Fit: Standard Not too tight, not too loose. Fabric Thickness: Look cool without breaking the bank. Our durable, high-quality, pre-shrunk100% cotton t-shirt is what to wear when you want to go comfortably casual. Preshrunk, durable and guaranteed. 5.6 oz. 100% cotton Standard fit