I saw this pin and knew the boys would love it--we just had to try it. And my realization that we already had M&Ms just sealed the deal. This pin was essentially the easiest I've completed. All you need is a flat, shallow dish (we used a tupperware container), a few M&Ms, and warm water. Fill the dish with enough warm water to cover the M&Ms, put the M&Ms inside ("m" side up), and wait for the magic. Go try it--this experiment is worth a few non-edible M&Ms.
Waiting for the magic to happen.
Sadly, Reed bumped the tupperware literally 30 seconds before I took this picture, which caused all our floating M's to break into pieces. But if you look toward the left, you can see a piece of one of the M's.
And because my picture of our poor, broken M's doesn't do the experiment justice, here is a picture from the original pin: