LET'S TRY FOLDING A PAPER CUP!!
1. First of all, you fold a square piece of paper from one corner to the other corner to form a triangle.
2. Fold the two bottom corners to the center to overlap.
3. Fold the top of the triangle outwards.
4. Then open the center.
Now you've got a paper cup!!
Origami around us "cups"
When you fold the balloon, you will see the change of the shape from the square to the hexahedra.
Or, you can see the change, for example
from right triangle to tetrahedra
from hexagon to octahedra.
These are the shapes that you learn in geometry