The police want you to confess. It makes their jobs a lot easier. As such, they'll often fish for confessions by asking simple questions and trying to catch you off guard. They hope that you'll be flustered and simply tell them what you did so they don't have to figure it out for themselves.
For example, at the beginning of most traffic stops, the officer will ask if you know why he or she pulled you over. It sounds like the beginning to a casual conversation, but it's not. It's their first step to getting you to simply admit what you did, perhaps as you try to make an excuse and hope you get out of a ticket.
Perhaps the officer saw you driving five miles an hour over the speed limit, for instance. When you're pulled over, though, you may not know you were speeding, and you could end up stammering out some excuse about how the car is yours and it is registered, but you just forgot to put the sticker on your license plate. The officer may not even have noticed that you didn't have the sticker in place, but now there's another violation to discuss.
It could keep spiraling. If you act nervous while talking to the officer -- something that is quite common and understandable -- and you can't find the registration papers, the officer may decide he or she thinks the car could be stolen. You could then be asked to step out while the car is searched. If the police find drugs or controlled substances in the car during the search, a simple traffic stop has now become a drug bust.
Remember, you have a right to remain silent, and you don't have to talk to the police without a lawyer. Always keep your rights in mind and don't let the police push you into doing something that breaches them.
Source: Alternet, "7 Ways Police Will Break the Law, Threaten or Lie to You to Get What they Want," Larken Rose, accessed Sep. 07, 2016