Take a look at this cart. It's moving to the right as it comes to a stop. What is the direction, if any, of its acceleration? Mr. Van Houten, what do you think?
"Um... right. NO! LEFT!"
Okay, which one? Right or left?
"Oh, so I guess it's right. Right?"
Mr. Van Houten... don't play horse games with me, man! Let's figure this out the right way so you don't have to guess.
In the early 1900s, word spread around Germany that a special horse named "Clever Hans" could solve simple arithmetic problems. He would answer by stamping his foot the correct number of times. Amazing! Even skeptics couldn't figure out how Clever Hans kept getting the answers right. They made Hans's trainer go away when they asked the questions -- but the horse still stamped the correct answers, so no one was cheating. Either the skeptics had to acknowledge a horse with mathematical ability superior to the average politician, or else something unknown was afoot.
Hans had essentially unfettered success until he was asked questions by people he couldn't see; then he failed. Hans also couldn't answer questions asked by people who didn't already know the answer. Hmm.
The logical conclusion was that the horse was extraordinarily attuned to peoples' body language. Those who asked Hans questions would tense up as Hans approached the correct answer... then they would sag with some emotion -- relief, for most, or resignation, for skeptics -- when Hans would make the final hoof tap.
For many physics questions, your students could have an easier time getting the right answer than even Clever Hans would. The choices are often limited to "right," "left," or "neither." Or perhaps "greater," "less," or "equal." In any case, students who equate physics understanding with merely choosing the right answer -- or students who just want the teacher to shut up and stop talking to them -- can read the teacher's body language to nudge them in the correction direction.
Now, I may not be a good poker player with cards, but I'm a pretty good physics teaching poker player. No matter my student's answer to an in-class question, I keep my face flat, and ask "why." We establish early on that trying to read my face does no good. I look the same for a wrong answer as a right answer, waiting for the explanation. It's impossible to feign understanding.
When, inevitably, a student enters into the dialogue described above, I tell him -- and the whole class, if possible -- the story of Clever Hans. "Don't play horse games with me. Physics is not about getting the right answer by any means necessary. It's about explaining the natural world using the facts and equations we've learned. It's about communicating clear predictions about experiments, then verifying those predictions with equipment in the lab. It's about refining those predictions when they turn out to be incorrect.
"But reading my face to get the right answer so I'll shut up? That's not physics. That's not even possible, as you just discovered. So don't be Clever Hans. Just answer the question as best you can, and if you're wrong, that's okay, you'll learn something."