[beznet]

In the video at the bottom of that link, what is inside of that rubber balloon that causes it to become firm once air is (presumably) removed?

[the_gastropod]

It reminds me of the Stretch Armstrong's nemesis toy, Vac Man: (https://www.youtube.com/watch?v=xkj9ptYsjTg)

It's probably small bits of foam or maybe even sand. When there's air in the balloon, there's room for the sand to move freely, and it's "soft". When the air is sucked out, the sand has to move past other sand, which is a lot of friction to work against, so it's firm.

[Robotbeat]

Google search term: "granular jamming"

[AWildC182]

Traditionally coffee grounds but I imagine a lot of granular substances would work reasonably well.