Here's a trick cops have:

They will ask/demand you sign a paper waiving your rights. NEVER SIGN THAT PAPER!!! Even if they threaten you with arrest or whatever, never waive your rights. If you do, then:
-the cops can arrest and hold you at their discretion, even if they don't have probable cause
-you do not have the right to keep silent. IOW, you must answer all their questions.
-Basically, the cops can do anything they want at that point to you.

I've seen cops use that move at least twice.