One of my favorite books I have read recently is titled "what cops know". The author interviews Chicago police who offer their insight on vice, gangs, drugs, organized crime, etc. One office speaks of when he first started that he was cruising with a seasoned cop, and he spots someone on street, and immediately pulls over and shakes them down- and finding a gun. The rookie asked "how did you know? "Its just something you learn". Twenty years later, that rookie became a seasoned cop and was riding around with a rookie. Well he spots someone on the street and immediate shakes them down- finding a gun. "how did you know?"