2D shapes have specific attributes: sides (how many), corners (vertices), straight vs. curved edges. A square has 4 equal sides and 4 corners; a circle has no sides or corners. Understanding these properties helps classify and identify shapes.
You already know how to recognize shapes like squares, triangles, and circles by sight. Now we go one step deeper: instead of just recognizing a shape by how it looks overall, we describe it by its properties — the specific measurable features that make it what it is. This matters because two shapes can look different but have the same properties (a tall thin rectangle and a wide short rectangle are both rectangles), and learning to read properties rather than just outlines is the foundation of all future geometry.
The two most important properties right now are sides and corners (also called vertices). A side is a straight line that forms part of the shape's boundary. A corner is where two sides meet. Count them carefully: a triangle has 3 sides and 3 corners; a square and rectangle each have 4 sides and 4 corners; a pentagon has 5 of each. These numbers are not coincidences — in any shape made of straight sides, the number of sides always equals the number of corners. That's a pattern worth noticing.
Circles are the important exception. A circle has no sides and no corners — its boundary is one continuous curve, never a straight line, never a point where two lines meet. This is what makes a circle different from all the polygon shapes. If you trace a circle with your finger, you never hit a corner or a straight section. That's the property test: are there corners? Are there straight edges? A shape with zero of each is a circle.
Once you can describe a shape by its side and corner count, you can figure out what a shape is even if you've never seen that exact version before. A shape with 6 sides and 6 corners? That's a hexagon. You didn't need to be told — the number told you. This is what mathematicians mean when they say properties define shapes: the shape's identity is its property list, not its size or color or the direction it's pointing.