Designer Hideyuki Kumagai created this Leaf Thermometer, which uses an undisclosed type of materials science to pull off a neat trick: It changes color with the temperature.
When it’s a comfortable 20 to 25 degrees Celsius (68 to 77 Fahrenheit), the Leaf is green; hotter than 25 degrees it turns a wilting yellow, and when it dips below 20, an ailing brown.
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Made from paper, the Leaf Thermometer comes in two sizes: A five-pack of Large leaves (65mm x 167mm, 2.6″ x 6.6″) or an 8-pack of Small leaves (43mm x 112mm, 1.7″ x 4.4″). Adhesive on the back allows you to stick it to a window or on a co-worker’s back.
Source: core77