From, well, the tail of an ox. It couldn't be any simpler. Quora probably will tell me this will collapse because it's too short, so: it's very common in Brazil's countryside. It's fat (too fat for me, not my favourite food), but healthy.
We can divide octagon into six triangles. So, the sum of the angles of each triangle is 180 degrees. For octagon, the sum of the interior angles is 1080 degrees and there are eight sides. Let us make a circle in the middle of the octagon and measure the central angle of a regular octagon. A circle is 360 degrees around.
That means each of the other angles in the triangle are 70 deg. Since the corner of the nonagon is the sum of the angles of the two triangles that make up the corner, the interior angle of a nonagon is 140 deg. Therefore the exterior angle of a nonagon is (180 - 140) = 40 deg. Each interior angle of a regular nonagon is 140 degrees.