There are two balloons. You inflate them to different sizes, bigger and smaller. Now, what will happen if you connect the mouths of those two balloons? Guess!

This is a relatively famous problem called ``Two-balloon experiment''. The result is actually a little counter-intuitive.

In this article I explain what happens and why, first based on the law of physics in Section 2, and then present more intuitive interpretation without using equations in Section 3.

I have found a few related documents about this problem over the Internet in English (and Japanese), but none of them was satisfactory to me. So, here you go. Enjoy!