No air

I caught a few of my coworkers off guard with this scenario. Take a balloon and fill it up with air. Now imagine you could freeze the balloon's structure. No matter what you do to it the balloon will always maintain it's inflated shape. I know it is physically impossible, just accept it as a truth. Now suck all of the air out of the balloon so that there is a total vacuum inside. The balloon will float away. It will be less dense than the surrounding air. So it will be pushed up, like an air bubble under water. It will float away even more rapidly than if it were filled with helium or hydrogen.

Right now, as far as I know, that scenario is not possible. But soon there will be a material strong enough to hold its shape with a vacuum inside, yet still be light enough to float. Perhaps it will be made out of nanotubes or something. I'm sure materials science will provide it sometime soon. Remember, it wasn't that long ago when we didn't have helium filled balloons.