Here is something you can do on your own, if you have a pencil and
a scrap of paper:
-- Draw a triangle. It has 3 sides. Count its angles.
-- Draw something with 4 sides. Count its angles.
-- Draw something with 5 sides. Count its angles.
-- Draw anything with 6 sides. Count its angles.
This doesn't PROVE anything, because you haven't tried polygons with
every possible number of sides, so there may be an exception to the rule.
But by trying only these few cases, you'll probably begin to notice a rule.
If you can't find a pencil and a scrap of paper, you can probably perform
this same experiment using a brain.
