Compose shapes to form others. For example, two right triangles can make a rectangle. Two right triangles can also make an isosceles triangle. Give kids some polyominoes, such as the T tetromino, and ask them to make a square or rectangle. Lots of puzzles can be tried at this point.
What manipulatives are available to buy? Maybe hand2mind has some.
Composing shapes by Khan Academy
Compose shapes by Khan Academy
Conclude by giving your students this challenge:
This challenge is about making squares out of rectangular pieces of paper, whether they be US letter or otherwise. You're allowed to fold the paper, draw with a pencil, and cut using scissors.
Part 1: How can you make a square from a single sheet of paper?
Part 2: How can you make a square from two sheets of paper, without folding either?
K.G.B.6: Compose simple shapes to form larger shapes. For example, "Can you join these two triangles with full sides touching to make a rectangle?"