How to make a prism bottle shape? ( not quite but I use this as learning opportunity)

Yes, we do have one.... an almost perfect prism bottle shape... ! How did we do it?
We'll it's a secret.

We went to Mt. Evans last week with a guest. The view was spectacular. Despite  having hard time with altitude sickness, I decided to the short hike to the top of the mountain and so did our daughters. As usual, we would bring extra drinks including a bottled water for every one.

After we got home and cleaned up the car, my younger

daughter found this odd shape bottled water. She was very curios and wondering how did it happen. I have a light bulb moment to use this opportunity to teach her about air density.

As we go to a higher altitude ( Mt Evens is about 14265 ft while Denver is 5280 ft), the air is less dense because the gravity pull the air downward towards the earth. When we opened the bottle at 14265 ft, the air density inside the bottle is the same with the air density at 14265 ft. When we go to a lower altitude where the air density is more, the higher density air pushes the lower density air. But why prism shape? This one I wasn't so sure as we only have one type of bottle water,  but I am guessing because triangle is the very stable shape of other shapes.

And of course, I have to do a little drawings when I talked to my daughter so she could understand easier ( I hope :) I seemed that my answer satisfy her curiosity. What do you think?