Suppose you’re driving a car on a perfectly level road at a perfectly constant velocity (because in physics problems, everything can be done perfectly unless the error is part of the problem). And suppose someone left a helium balloon inside the car, which is free to float around. Similarly, suppose someone left a ball resting on the floor of the car; since there are no jolts or other sudden accelerations, the ball is resting perfectly still. Now suppose you start accelerating forward. What happens to the ball and the balloon? Our physical intuition tells us that the ball will roll to the back of the car, because that’s what happens when we’re in an accelerating vehicle; things get pressed back. But what does the balloon do?

A lot of people, when presented with this, intuitively think that the balloon will move towards the back, just like the ball. If they’re physics-minded, they might say that the inertia of the balloon keeps it from wanting to change its velocity unless it’s forced to, and the only thing that can force it to do so is the back of the car. But if this were true, then general relativity would be wrong! Because according to general relativity, if you don’t look outside the car window or cheat in some other way, there’s no way to tell between a uniform acceleration of 9.8 meters per second per second in the forward direction and an additional gravitational force of one G pulling you back. And when we think about it in terms of gravity, the answer becomes clear: balloons in air will move against gravitational fields, so it will move to the front of the car.

Hopefully, this has left you at least somewhat unsatisfied; the question of where the balloon moves has been answered, but not why it moves. For that question, consider why balloons move against gravitational fields. They move because of buoyancy, which is a result of the fact that pressure is greater when you go ‘deeper’ into a fluid (liquid or gas) in a gravitational field. So the upwards force as a result of pressure is greater than the downwards force, and the balloon rises until something stops it or the atmosphere gets so rarefied that it reaches zero buoyancy (or, more likely, the decreased atmospheric pressure causes it to pop!) But when you have an accelerating car, the air molecules, much like the rolling ball, will tend to ‘pile up’ in the back, causing greater pressure in the rear than in the front. And in general, this pressure gradient will be enough to cause a forward force on the balloon all the way to the front of the car.