‘Baffling’ Balloon Experiment

Science in Practical is always better than written theories on books.
In this, the latest installment of the outstanding YouTube science series Smarter Every Day, the host Destin explores the baffling behavior of a helium balloon in an accelerating vehicle. What's so baffling about it? See for yourself:

What's remarkable about this experiment is that the balloon's behavior seems completely counterintuitive to what we've been conditioned – with every turn, brake, and press of the gas pedal – to expect of objects in accelerating vehicles. 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, as Destin alludes to with the mason jar full of water. Air is less dense than water, and is readily displaced by the water's motion in the mason jar. Similarly, helium is less dense than air. When the car accelerates, the air molecules in the car resist the change in motion (thanks inertia!) and collect in the back of the vehicle's interior. The air displaces the helium-filled balloon and causes it to move forward, away from the rear of the car where the air molecules have collected.