That is a tough one. Not all of my students really get to this one in a genuine manner to really start asking those questions but for the ones that do, I say this; What do you think? Are there multiple ways to think about this problem? Are there different cases that would warrant a different solution? I try and push them back to the define step of the problem solving process to hopefully really start to see that defining a problem can be a big part of the process. And ultimately, hopefully they will come to their own understanding of what the problem is asking.
This is also a good place to start to show the students that the teacher does not always know they answer and/or is not going to just give them the solution. Students need to focus on creating their own meaning and solutions and sometimes we won't reach that the first few times through.
Also, I would say that the lesson is not about getting to the answer, it it about processing, and coming up with that process, or trying to get to that process is more what the lesson is about which can leave some students hanging.