Look at these together: a flat triangle, a flat square, and three solid shapes shaped like a cereal box, a tin of beans and a triangular chocolate packet. What is the same about them, and what is different? If you had to put them into groups, what would you sort them by?

The interactive reveals each shape one property at a time: sides, vertices, parallel sides and fold-lines for the flat shapes, then faces, edges and vertices for the solids. Watch closely and count each part as it lights up. A fold-line, or line of symmetry, is a line you could fold the shape along so both halves match exactly.

Now we try it together. The whole class predicts first, then the teacher brings a pupil up in turn to reveal and check — your job from your seat is to predict and agree, not to come up unless you are called.

For the flat shape on the board, call out its sides, its vertices and its lines of symmetry, then a pupil reveals each property to check. Next we look at a solid shape, rotate it on screen, and count its faces, edges and vertices. Count along with the rotating shape on screen — and if your group has a box or polydron solid, count round that too.

Today we sort a mix of shapes into groups. First we sort just the flat shapes by how many sides they have. Then we sort just the solids by how many faces they have. Next we group the flat shapes by how many lines of symmetry they have, and the solids by how many vertices they have.

Today we learned

• Flat (2D) shapes are sorted by sides, vertices, parallel sides and lines of symmetry.
• Solid (3D) shapes are sorted by faces, edges and vertices.
• Shapes with the same number of sides can still be different — symmetry and parallel sides tell them apart.