Counting Sides and Corners to Tell Shapes Apart

Look for this idea outside of your lessons. Builders and tile setters count sides and corners constantly, because a piece with one extra corner will not fit. This is worth learning because counting beats guessing when two shapes look similar.

Watch for it today, and when you spot it, tell someone what you noticed and how you knew. Ask yourself where else the same idea might be hiding, then go and look.

Let us think about this slowly. Sides and corners identify a shape better than a quick glance does. Put your finger on one corner and count around the outside, touching each side once. A square has four sides and four corners.

A triangle has three of each. Notice that flat shapes always have the same number of sides as corners. Counting carefully protects you from mistaking a trapezoid for a rectangle when it is drawn quickly.

Try counting with a marker. Put a small dot on each corner as you count it, so you cannot count the same one twice or lose your place halfway around the shape. Show your work to someone and explain it in your own words.