Its the way that they smile - actually more of a smirk than a smile.

At the office where i used to work a woman at the far end was always smirking every time we crossed paths. I would always politely smile back (result of being brought up by strict parents who were a stickler for good manners).

One morning when queuing up at the sandwich trolley i was on the verge of asking her but she must have sensed it, and she asked me

"Is it true you were once a cop?"

"Yeah", i said , "How did you know that?"

"Cos you still walk like one, tis why me and my colleague are always giggling at you!"

Charming huh!