Cut out colored paper triangles (rectangular, isosceles with them more interesting to work). Ask your child to put their "growth" (as squares), apply one to the other, to determine which of them anymore. And now let the kid try two colored triangles to form a square. Now of the four triangles. Help him, if necessary. Maybe he will come up with some more figures. The most beautiful glue it to the paper.

The remaining triangles are put in an envelope together with the squares.