The Stability of a Triangle

The three-legged stool/chair should have felt more stable than one with four legs

Why is a three-legged stool more sturdy?

It seems strange because you would think that more legs on a stool or chair would make it more sturdy or stable but in fact not!.

What geometry concept can be used to explain this?

The reason actually lies in the fact that it only takes three points to define a plane.

Any point that is added to that plane will make it harder and harder for the plane to be stable.

Here's a great activity to show this concept:

Your fingers will represent points and the paper will represent a plane.