Fold and Sort
Would it fold in half exactly? Then sort the triangles by their corners.
For grown-ups
- How long: 10 to 15 minutes is plenty. Short and frequent beats long and occasional — the early-numeracy trials with the largest effects ran eight weeks or less.
- When they get one wrong: the page shows the working. Read it together rather than moving straight on. Feedback that explains is worth several times more than feedback that only marks right or wrong, and that gap is wider in maths than in any other subject.
- Not timed: thinking time is the point of a book. Let silences run.
- Print it too: doing the same problems on paper a few days later is worth more than doing twice as many today.
What this practises
Finding lines of symmetry by folding, and sorting triangles by their angles.
Geometry was the largest hole in the catalogue: 4.G.A.2 and 4.G.A.3 appeared nowhere, and IM grade-4 Unit 8 had no activity at all. Stated plainly, because the site does not claim what it cannot support: there is no efficacy evidence behind this one. references.js holds 37 sources and not one is geometry; the WWC practice guide that touches it rates geometry Minimal and is a preschool/K guide. im-k5 is cited as a design source for the sequence, not as evidence of effect. What this activity is defended on is coverage, and one affordance paper has and screens do not — you can fold the sheet and find out.
- The underlying idea
- A line of symmetry is defined by a fold, so the test is physical rather than visual. Shapes are drawn off-axis because a child who has only met upright figures learns "upright" as part of the definition.
- Where this sits in Illustrative Mathematics
- 4th grade Unit 8: Properties of Two-dimensional Shapes
- Sources
- Illustrative Mathematics (2024) · Illustrative Mathematics / Kendall Hunt (2024) all references →
Related measured skill: ALPACA · Elementary operations. What this means