actually, it should be asked whether they breathe at all or not. they do, according to their own belief while in reality, whether they breathe or not is a mystery. you can tell them that the space inside a normal box is a vacuum, and they will suffocate on normal air in that box. the converse experiment should also be true: tell them that a solid jar is filled with air in all cases, and run a vacuum packaging machine on it - the yukkuri's should still be fine, aside from their eyes popping out and their bodies exploding due to depressurization.