Exterior Angles of Polygons

The Exterior Angle is the angle between any side of a shape,
and a line extended from the next side.

Another example:

When we add up the Interior Angle and Exterior Angle we get a straight line 180°. They are "Supplementary Angles".

Polygons

A Polygon is any flat shape with straight sides

The Exterior Angles of a Polygon add up to 360°

In other words the exterior angles add up to one full revolution.

Press Play button to see.

(Exercise: try this with a square, then with some interesting polygon you invent yourself.)

Note: This rule only works for simple polygons.

We can also think "each line changes direction
until we get back to the start again":