Reason: The tightness of the plug depends on the friction between the plug and the hole.
Let's first look at a round bottle stopper inserted into a square hole. Because the bottle stopper is deformed and the bottle mouth is square, it can't be screwed in by rotating, otherwise the bottle stopper is still round, which means the friction is very small. If you want to plug the stopper deeper, your only way is to squeeze it in the direction of the bottle, and there is no other way. In this way, you think that the plug is deformed by pushing, and the outer part of the plug becomes thicker, which makes it more difficult to insert. Because you don't insert it deeply, the plug is not tight.
Let's look at the situation where a square is inserted into a round hole. Because the bottle stopper is deformed into a round shape, and the bottle mouth is round, you can screw it in by rotating, and at the same time, the stopper will become thinner, reducing the extrusion with the hole and reducing the friction when screwing in, thus easily screwing the stopper into the hole deeper. When you let go of your hand, the plug will expand because of its own reversal, so it will be more tightly stuck, so it is not easy for you to pull it out. Therefore, the method of stuffing the square into the round hole is more rigorous!
Another reason is that the square hole is easy to expand the hole area under external force. Moreover, the circle is the smallest circumference under the same area, and the area of the hole is not easy to become larger. So it's tighter.