A diagonal is from one corner of a shape to another corner.
Here is all the diagonals of a pentagon: As you can see it looks like a five point star because there are five corners so a ten sided decagon will make a ten point star by connecting all the diagonals
You might be thinking what about a circle? A circle has infinite diagonals, but you can actually make any point star with a circle for example you can create a ten point star by doing this;
A circle is a 360o angle so if you want a ten point star you have to;
Divide 360 by 10 = 36 now use the protractor and measure the angle of 36 degrees starting from a line drawn from the of the circle. Each time you measure 36 degrees draw a new line and then measure the degrees again.
This is my 18 point star some parts got messed up but it was pretty hard to make;
