"Food for thought"

I guess all, if not most of us has taken an air flight before...but have you ever wondered how we actually survive that flight?

We all know that the air cabin is pressurised, about the pressure which is 2000m above sea level. This is so as prevent the plane from being crushed by pressure. But...at 2000m above sea level, most mountainers already need oxygen tanks while they are hiking. However, why do we not need oxygen tanks even though we are in the same scenerio as them?

Anybody knows the ans? I would like to hear it too...see if anyone thinks the same view as me...and my friend...
Btw...this question was posed by my physics teacher...he doesn know the ans too :P

Happy weekend!