I have relatives who ask me that. "If electricity moves at the speed of light, why does it take so long to download webpages?" and then I have to explain latency vs. bandwidth and blah blah blah. Water pressure analogies seem to work pretty well.
You should be glad that your relatives ask such cogent, nearly relevant questions. The most I ever get is; "I just bought this brand new computer, the guy told me it's super fast, and I still can't upload videos to facebook any faster! Why is that?" Your family are some serious brains, in comparison.
My dad learned Fortran in college yet it took him at least 6 years to understand the Windows recycle bin and he still doesn't get basic keyboard shortcuts.
I'm pretty sure that they just dropped every other frame in the video and there aren't actually any black frames in the video at that point. Note the wording, "if we could speed the film up to 48 frames per second" - which tells me that they actually were stuck at 24fps, because this was probably an actual film.