The window of enjoyability for hot beverages is pretty small: you've got about 30 seconds of 'just right' between tongue meltingly hot and disappointingly lukewarm. Giant magic beans with liquid metal inside use the power of physics to simultaneously make hot things cooler and cool things hotter, prolonging that ideal temperature.
Coffee Joulies are stainless steel coffee beans with a special phase change metal inside of them. 'Phase change' just means that it's a metal that goes from solid to liquid and back again, but doing that is a lot of work, and the metal sucks up a substantial amount of heat when it melts. This means that when you drop one of these beans into hot coffee, the bean's insides will melt and the coffee will get cooler.
It gets better, too. The melted nugaty centers of those beans will slowly release their pent-up heat back into your coffee after they've melted, helping to keep it from getting all cold and gross. The upshot of all of this phase changing is that Coffee Joulies can cool your coffee down to a comfortable 130 or 140 degrees Fahrenheit three times faster, and then keep it at that temperature for about twice as long. And the better insulated your container is, the more effective the beans are.
If you're picky about your temperature of maximum enjoyment, you can add or subtract beans to fine-tune their cooperative performance. By the way, yes, they're shaped like coffee beans, and they're sort of supposed to be for coffee, but they'll work on just about anything that needs to stay warm besides a vaguely gross-tasting brown sludge that makes people nutty.
Coffee Joulies are currently on Kickstarter and have unsurprisingly blown way past their funding goal, but there's still plenty of time to pre-order a set of five beans for $40.