Visual proof 

Here’s a triangle.  A B and C are internal angles.  D E and F are external angles.  Here is a nice visual proof of some of the theory we’re working on.

The external angles, have a sum of 360 degrees.  Here’s proof.

When we put them all together, they match up perfectly, completing the full circle, which is 360 degrees.

Also, when we put the interior angles together, they make a straight line.  They are supplementary.  They add up to 180 degrees.