How can (someone) be so (adjective)?

If someone does something that makes you angry, and is also hard for you to understand, you can say something like this:

How can you be so stupid!?

How can he be so arrogant?

This is a phrase that you say when you're in an argument, or when you're complaining about someone. Don't use it when you want to seem helpful or patient.

This phrase appears in these lessons: