What is this skill?
In first grade, children move from naming shapes to describing what makes each shape what it is. They count sides (the straight edges) and corners, also called vertices (where two sides meet). A triangle has 3 sides and 3 corners, squares and rectangles have 4, a pentagon has 5, a hexagon has 6 and an octagon has 8.
Children learn the difference between defining attributes, which decide what a shape is, and attributes that do not matter. A triangle must have three straight sides and be closed, but its color, size and which way it is turned do not change what it is.
Why it matters
Describing shapes by their properties is the beginning of mathematical reasoning about geometry. It leads to classifying shapes in later grades, for example understanding that a square is a special kind of rectangle, and to measuring perimeter and area.
Counting sides and corners also builds precise vocabulary and careful observation. Children have to look closely and count systematically, which is a useful habit in every part of math.
Composing shapes is a related first grade idea: putting shapes together to make new ones, such as two triangles making a square or a rectangle, or six triangles making a hexagon. Children who know the sides and corners of each shape find it much easier to see how they fit together, which also prepares them for halves and fourths, where a shape is split into equal parts.
Common mistakes
- Counting the same side twice or missing one when going around a shape. Starting at a marked corner and touching each side in turn solves this.
- Thinking a shape changes when it is turned or resized, such as saying a tilted square is a diamond and therefore not a square. Rotating cut-out shapes while counting sides shows that nothing important has changed.
- Confusing sides and corners. Showing that a side is a straight line and a corner is a point where two lines meet, and noting that the counts match for these shapes, helps.
How to help at home
- Go on a shape hunt: a stop sign is an octagon, a slice of pizza is roughly a triangle, many tiles are hexagons. Count the sides of each shape you find.
- Build shapes with straws, toothpicks or craft sticks and modeling clay for the corners. Ask how many sticks each shape needs.
- Draw shapes with a ruler and let your child count the sides and corners, marking each one with a dot as they count.
- Play "guess my shape": describe a shape by its attributes, such as "I have 5 sides and 5 corners," and let your child name it.
Worked examples
These are real questions from the Tutorsaurus sides and corners of shapes practice, with the answers worked out.
Example 1
How many sides does a rectangle have?
Choices: 5, 6, 7, 4
Answer: 4. A rectangle has 4 sides and 4 corners.
Example 2
How many sides does a square have?
Choices: 4, 3, 7, 5
Answer: 4. A square has 4 sides and 4 corners.
Example 3
How many sides does a pentagon have?
Choices: 5, 6, 8, 4
Answer: 5. A pentagon has 5 sides and 5 corners.