Nets and solids: the miscount
A solid is described in words or shown as a net, and the child has to count faces, edges or vertices, or say which solid folds from the net. The common slip is counting only what can be seen, or mixing up a prism (two matching ends) with a pyramid (one base and a point).
What it looks like
A prism has a triangle as its base. How many edges does it have?
- The top triangle has 3 edges and the bottom triangle has 3 edges.
- 3 upright edges join the two triangles.
- 3 + 3 + 3 = 9
Why 6 is wrong: Only the two triangles were counted. The three edges joining them were missed.
Why children do it
- A drawing shows at most three faces of a solid; the hidden ones have to be imagined.
- Edges are counted as "sides of the shapes I can see", so the ones joining the two ends are missed.
- Prism and pyramid names both start with the shape of the base, which makes them easy to swap.
How to fix it at home
- Use real boxes: a tissue box, a Toblerone box, a tent. Count faces, then edges, then corners, touching each one.
- Teach the pattern: a prism with an n-sided base has n + 2 faces, 3n edges and 2n vertices; a pyramid has n + 1 faces, 2n edges and n + 1 vertices.
- For a net, have your child name every shape in it before deciding which solid it makes.
Questions parents ask
Which solids are in the PSLE?
Cubes, cuboids, prisms and pyramids, including their nets. Nets of prisms and pyramids are the P6 item.
Does my child need to draw nets?
Identifying a net, and the solid it folds into, is what is tested. Drawing is not required in the paper.
How does Knowbotic catch it?
Miscounts and prism-for-pyramid swaps are known traps on solids questions, so a wrong answer is named as this habit and practised until fixed.
Where it shows up
Other careless mistakes
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