Why do people recommend separating drinking glasses that are stuck together by cooling the inner glass and heating the outer glass?

Question

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?

In: 0

Answer 1

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.