Children describe 2D shapes by counting and naming their sides (straight lines) and corners (points where sides meet). Understanding attributes helps classify and compare shapes.
You already know how to recognize shapes by sight — a circle looks round, a square has four equal sides, a triangle has three points. Now we go one step further: instead of just recognizing shapes by what they look like overall, we describe them by their attributes — the specific parts they have. This is how mathematicians think about shapes.
The two main attributes of flat (2D) shapes are sides and corners. A side is a straight line that forms part of the shape's boundary. A corner (also called a vertex) is the point where two sides meet and change direction. Counting these parts gives each shape a mathematical "fingerprint." A triangle has 3 sides and 3 corners. A square has 4 sides and 4 corners. A rectangle also has 4 sides and 4 corners (but its sides are not all the same length). A hexagon has 6 sides and 6 corners.
A circle is special: it has no straight sides and no corners at all. Its boundary is one continuous curve. That is why we say a circle has 0 sides and 0 corners — and it is what makes a circle different from every other common shape. This is an important observation: shapes can be sorted by how many sides they have, and a circle forms its own group.
When you know the attributes of a shape, you can talk about it precisely and compare it to other shapes. A square and a rectangle both have 4 sides and 4 corners — so what makes them different? The length of their sides. A square has all four sides equal; a rectangle can have two long sides and two short sides. Attributes let you notice both what shapes share and how they differ, which is the foundation for classifying and sorting shapes as you continue to study geometry.