A polygon is a flat, closed shape made up of straight lines connected end to end.

Triangles, squares, rectangles, pentagons, and octagons are all polygons. Each straight line is called a side, and each corner where two sides meet is called a vertex (plural: vertices) or an angle. The simplest polygon is a triangle with three sides, but polygons can have any number of sides. A stop sign is an octagon (eight sides), while a honeycomb is made of hexagons (six sides).

For a shape to be a polygon, it must follow some rules: all sides must be straight (so circles don't count), and the sides must connect to form a completely closed shape with no gaps. Polygons with sides of equal length and angles of equal measure are called regular polygons. A square is a regular polygon, but most rectangles are not because their sides aren't all the same length.

Mathematicians name polygons based on their number of sides: pentagon (5), hexagon (6), heptagon (7), and so on. After about ten sides, polygons start looking more and more like circles, even though they're still made of straight lines.