Why can't we breath under water?

We can certainly draw water into our lungs, even though

our brain screams "Don't do that!".

But our lungs can only separate oxygen out of air, not out of

water. So if there's only water in our lungs, we pass out after

a short time because we're not getting any oxygen.

Our lungs are not designed breathe oxygen from water to be able to breathe under water.