Duh!

"Duh!" is a sarcastic interjection used to point out that someone has said or done something painfully obvious or stupid.

Examples:
-I really need to study for that physics final if I'm going to get an A!
-Duh!

-Now where did I leave my car keys? Oh, duh, they're right here in my pocket!


True story:
I'm surprised at how often I find myself saying this, especially to myself!