Can someone tell me how to not look dumb?

So if someone says something sarcastic, and I don't pick up on it, and they correct me, how do I not look dumb? Normally I say "I know" real dead-pan and straight-faced and that just makes it worse.

For example:

Person: Sally-Jo Rae is 34, she just hides it really well.

Me: Wow, I had no idea. She must use some awesome women's product to stay young.

Person: I'm joking. She's actually 23.

Me: I know... '_'

So what could I say there instead so that I won't look like a total moron?