"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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PB Condensation Nuclei Women's Long Sleeve T-Shir Powered By Condensation Nuclei: Ok, you know them as clouds, and maybe you're the kind of person that loves rainy days, but its fun to confuse everyone with technical terms anyway. $23.99 Size:  Size Chart Small Medium Large X-Large 2X-Large (+$3.00) Qty:    AVAILABILITY: Product Number: 118047830 Tell a friend about this product! Product Information Fit: Standard Not too tight, not too loose. Fabric Thickness: Our 100% cotton, women's long sleeve tee is preshrunk, durable and guaranteed. Constructed of mid-weight jersey, it's just the right weight for layering. Side seamed construction provides a perfect shape, for a contoured but loose fit. 5.5 oz. 100% luxuriously soft combed ring spun cotton Standard fit Side seamed contoured shape