Two cans of beer with a damp towel wrapped around them. In theory, the sun heats up the water and the water evaporates. But to evaporate, the water needs to suck out energy from it's surroundings, however that works, and does this by robbing heat from the beer.
The towel is wrapped around the top container and wicks water from the container below. This way the towel doesn't dry up, and the beer is not surrounded by water that just gets heated by all that sun.
Might be hard to see on this picture, but why would I lie about something like this.
Astoundingly, this does work. and the principle here has been used all over the place for a long time.
I'm not saying that I ended up with near-freezing temperatures here. But it was sort of cool, in two ways at least.