Mountains and Valleys, over and under
Be careful you do not tear it asunder
Boats and planes, birds and flowers
Create almost anything with this power
What could it be? You'll have to guess
I hope you don't fold under the stress.
Mountains and valleys are different types of creases.
When you crease too hard, it becomes easy to rip.
You can create many shapes with folding paper.
"Don't fold under the stress" implies folding.