Students learn that a polygon is a closed figure whose sides are line segments that do not cross, and a regular polygon is a polygon where all of the sides have equal lengths and all of the angles have equal measures. Students also learn to classify polygons based on number of sides: triangle (3 sides), quadrilateral (4 sides), pentagon (5 sides), hexagon (6 sides), heptagon (7 sides), octagon (8 sides), and decagon (10 sides).