It’s happened to the best of us. It’s 10pm, the sun has long set and a complex math problem arises. You reach into your desk drawer and pull out your trusty old calculator. Only it’s solar-powered. Texas Instruments released their very first solar-powered handheld calculator in 1981—back when all you had to do to save the planet was recycle and plant a tree—but times have changed and now there are other eco-friendly ways to power your devices. We’ve looked around and Gordon Sinclair has a great Hydro Calculator. Any liquid will power this gadget (we recommend avoiding hot coffee). The company also offers a smaller, less sleek-looking, but still totally green and fully functional Water Powered Calculator as well as a Water Powered Alarm Clock (though, personally, we’re not sure we’d want to rely on easily evaporated H20 to get us up on time for work) or, if you want to be literally green, a Water Powered Clock with Flower Holder!