I have seen mutliple people recommend this. Here is why it confused me:
If you had a large balloon stuck in the center of an inner-tube, and you wanted to separate them quickly, you would want both the baloon and the inner-tube to contract toward their centers.
If the baloon was slowly deflating (by losing air or cooling down), and you wanted it to stay stuck, you would want the inner-tube to inflate (by adding air or heating up) in order to keep its grip.
So, why are people recommending to expand one glass while shrinking the other? Won’t that just maintain the grip by the outer glass? Don’t we want to cool both glasses, so they both take up less space, creating air between them?
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Things usually expand when heated and contract when cooled. If the outer glass expands and the inner glass contracts there will be more room between the inner and outer glasses. If you cool both then both contract and there isn’t any more space between them.
I see what you are thing. You think that blowing up the inner tube will increase its width but not the length. This is not the case. If you let air out of the balloon no matter how much you inflate the inner tube it will not prevent the balloon from falling.
Also if you kept the balloon at the same inflation, inflating the tube will not squeeze it more but will loosen its grip.
This is the same as the glasses. The outer one will get bigger in inside diameter and loosen its grip.
If you had a balloon stuck in the center of an inner tube, you would overinflate the tube (so that the void in the center got larger) and underinflate the balloon (so that its radius was smaller) and thereby free the balloon.
Essentially, the hole gets bigger faster than the inside edge can fill it in.
Here’s an Eli5 way to think about it: forget the inflation for a second, and think about growing or shrinking how big the tube _loojs_ just by walking towards or away from it. As you walk towards it, all aspects of the shape get bigger: the tube, but also the hole. If you have a tire from far enough away that your thumb just barely covers the hole, then as you get closer, your thumb will more easily fit inside the hole. Now, imagine making a video of that happening, and comparing it to a video where you stay still, but inflate the tire. The two videos would look the same, right?
The slightly less eli5 version is that the material in the inner part of the ring is expanding, and and basically pushing against itself in a circle. It doesn’t push inwards and become thicker, it pushes “sideways” (against its neighborhood material in that inner circle of tube) and creates a bigger circle, becoming thinner.
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