A pinboard is a great way to investigate shapes.
This may be made by nailing panel pins or small nails into a piece of plywood approximately 24cm × 24cm.
Cut out the plywood and mark a grid of lines at 2cm intervals in both directions across the plywood. Tap in the pins at the points where the lines of the grid meet. Leave enough of the pins protruding to accommodate elastic bands.
This page looks at how many different 3-sided shapes can be made. The results can be drawn onto the page of ‘pinboards’. When investigating this it is important to talk about the shapes and ask questions such as:
what counts as a triangle?
how many different kinds of triangle have you made?
Are some the same, but just a different way round?
etc