Blue triangles are a classic Montessori material. Learn how to use blue triangles to teach shapes in this free educational video clip about teaching visual skills with Montessori methods.
These are blue, constructive triangles. You start by taking them out, one at a time. Now you slide them together, trying to make different shapes. That is the blue, constructive triangles. They can make any shape with them that they wish. There's no right or wrong to this, as long as they know to treat them with care, and to just slide them along to make the shapes. When they're done, show them how to put them back in the box, one at a time. If they put them back correctly, they will fit. If they don't put them back correctly, the lid will not go on. That is the self-correct for this work.