Poor thing, doesn't understand sarcasm.  I was listening to her tell my brother that he shouldn't have his face so close to the screen of his iPod 'cause it's bad for his eyes.  So, I jokingly said "Oh, who cares about his eyes, his breath is probably ruining the screen!"  She snapped her head and gave me this disapproving, how-could-you-say-that, WHAT DID YOU SAY!? look.  So, then I said (as she was walking up the stairs) "I was just being sarcastic, you want me to hold up a sarcasm sign?"
I wonder if people are born either able to comprehend sarcasm or not able, or maybe's it's a developmental thing...either way, my poor mother still hasn't quite got the hand on sarcasm yet.  (And this is NOT the first time she's had issues with comprehension—or lack thereof—regarding sarcasm)