So let me be clear, I hate cops and think that they're class traitors.
That said, I had a really fun interaction with them earlier today.
While I was in front of my office a sketchy looking dude with a bunch of bags meandered over to a locked bike, pulled out a pair of side cutters, cut the chain, and rolled it around the corner.
So I grabbed all of his stuff, which he'd left on the ground, moved it inside my office's stairwell and locked the door.
He came back (sans bike) and demanded his stuff. I demanded the stolen bike. He gave me some flimsy story about it being his girlfriend's and I explained that it was a interesting story for the police, with whom I was currently on the phone.
At that point he left, as innocent people are wont to do in such situations.
Despite some of my coworkers worrying the legality of my tactics, the police congratulated me for my quick thinking as the dude's ID was in his bag.
That's my fun story of the day.