Characteristic numbers

Imagine you have a bunch of shapes, like circles, squares, and triangles. Each one of these shapes has something called a "characteristic number" that helps tell you what kind of shape it is. It's like a special code that only that one type of shape has.

For example, circles have a characteristic number called "pi" which is about 3.14. This number helps you figure out things like how big or how long a circle is, just by knowing how wide it is.

Squares have a characteristic number too! It's called the "side length" and it's just the length of one of the sides of the square. This helps you figure out things like how much area the square takes up, just by knowing how big each side is.

Triangles are a little more complicated. They have a few different characteristic numbers, depending on what kind of triangle you're talking about. For example, if you have a right triangle (one with a 90-degree angle), you can use something called the "Pythagorean theorem" to find its characteristic number, which is the length of the longest side (also called the "hypotenuse").

So, to summarize: characteristic numbers are like secret codes that help you figure out what kind of shape you're dealing with, and how to measure or calculate things about that shape.