Smile and start talking as my brain starts running around screaming in panic mode trying to figure out what to say. Realize I'm panicking over what should be an easy interaction which induces more panicking and nervousness. This anxeity triggers a biological reaction that flushes the cheeks and can't be hidden. Induce visible amusement and possibly pity in the face of my conversation partner as I fight the urge to break eye contact, mumble a closing line, and walk away.
I'd like to think I've gotten a lot better, and in a lot of ways I have, but if a really good looking woman walked over to me, said hi, and showed intrest? Once I worked past my suspicion you might as well slam my tounge in a car door cause it'd be about that useful.