What causes popovers to rise so much? I recently made popovers and I knew they would 'pop over'.
Yet, I was surprised they came that high.
I'm curious how this is possible, since there is no yeast, baking powder, self-rising flour, beaten egg whites...

I think it's because of the egg, but I'm not sure.
So, can somebody explain this?
There is a lot of water in the milk. The high heat turns the water into steam. The egg and flour contains proteins and gluten that forms a "net". The steam can't escape without pushing the net upwards. This makes the dough rise. 

It helps that the container is a narrow cylinder.