When I am about to ask my students a difficult question, I say, "And here's the AP Seminary question..." They know this means that I am about to stretch them a little bit. I think the little verbal cue helps the students buckle down and recognize that a serious question is coming. They up their game and are prepared to respond to these more difficult questions.
EXAMPLE: While teaching the Lord's Prayer in Luke 11, I asked "Have you ever prayed for something that you didn't get?" They all admitted they had. "Now here's the AP seminary question: doesn't that contradict the JST footnote for Luke 11:5a? Why not?"
Asking difficult questions helps students learn that there are answers to many of life's difficult questions. Sometimes we have to think through them a little bit more or ask a friend for help, but it's okay to ask hard questions.