A while back, in a Part 6 of my Revamping Intro Physics Lab series, I mentioned an "experiment" that students can do involving a suspended helium balloon in an accelerating vehicle. I mentioned that this would be an excellent example of something where the students get to guess what would happen, and at first, what actually happens does not make sense.
Well now, we have a clear demonstration of this effect on video.
There is a good explanation of why this occurs in the video. It is also nice that he included a hanging pendulum in the beginning for comparison, and that this is what most of us are expecting to occur.
Might be a nice one to quiz your kids if you are teaching basic, intro physics.