Possible Clues:

• 'Like, I get it'
• 'How could I be so dumb?'
• 'What a dope!'
• 'Of course, blockhead!'
• Sarcastic response
• Forehead-slapper's cry
• "How could I be so stupid?"
• 'Isn't that obvious?'
• Bonehead's response
• ''I knew that!''