A pentagon is a five-sided polygon with 540 degree internal angles. Regular pentagons have equal-length sides and 108-degree interior angles. An octagon is an 8-sided polygon having 1080 degree internal angles. Regular octagons have equal-length sides and a 135-degree interior angle. An example of a regular pentagon is shown below. The first pentagonal number is 5, which is the sum of 1+4=5 squares. The second pentagonal number is 25, which is the sum of 2+4+8=14 squares. Pentagrams were used in ancient Greece and Rome, where they were made out of wood. Modern inventions include paper, plastic, and metal.