Because the distance from the edge of the circular manhole cover to the center of the circle is equal everywhere, no matter how the manhole cover rotates, the manhole cover will not fall into the well. One side of the square is shorter than its diagonal, and once the manhole cover rotates, it may fall into the well.

2. Why do people naturally form a circle when they are watching?

Or because the radii of circles are all equal, when people form a circle, the distance between the center of the circle and everyone is equal, so that everyone can see clearly!

3. Why are the wheels of a bicycle round instead of square?

Because only circles roll along straight lines.

Both donated half of their pocket money. They donated the same amount of money, didn't they?

No, because the pocket money they have is unknown, that is, the corresponding overall is different.

5. Why are some iron shelves surrounded by rectangles or squares instead of parallelograms?