Probable Cause and Reasonable Suspicion
The two levels of evidence needed to detain, search or arrest.
Reasonable suspicion
A specific, describable reason to believe a person is involved in a crime. It allows a brief detention to ask questions or check identification.
Probable cause
Facts that would lead a reasonable officer to believe a crime was committed and that the person did it. It allows an arrest, and a search in many cases.
What does not count
A hunch, a person's looks alone, or refusing to talk.