Ed. Note: Due to the breakout success of i09's story on a balloon in a minivan, Nick Denton has asked each Gawker subdomain to blow its readership's mind with a balloon-related post. This is Fittish's version.
If you don't already know why a helium balloon tethered to a child running through a field has the power to make your jaw drop, you're going to want to see this. Seriously – set aside five minutes of your time, have a seat and look. You won't regret it.
What's remarkable about this experiment is that the balloon's behavior seems completely counterintuitive to what we've been conditioned to expect of objects placed on running humans. It doesn't play music, provide hydration, or contain a logo of any apparel maker. What's beautiful about it is that this behavior isn't counterintuitive at all.
It's not immediately obvious, but this is actually a fluid dynamics problem (runners have a lot of fluid problems). Helium is less dense than air, and air is less dense then most runners. When the runner accelerates, the air molecules in the air are really pissed off that someone's just running right through them, rather than yelling out "on your right" and passing safely to the side. The pissed-off air kicks the helium-filled balloon out of spite, and causes it to move upward.
Still confused? There's an even easier, conceptual way to wrap your head around this problem: