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Disclaimer: I teach at university, it is much different than teaching younger students (e.g. I wouldn't really know how to handle them well).

The approach that works best for me is to force them to answer and to do it so frequently as to make it natural for them to ask questions, while, at the same time, letting them know it's alright to make mistakes.

To given an example, I often ask questions like "what technique would you want to use" which, sometimes, I make deliberately hard. Then, after a bad answer I respond "unfortunately, this approach will not work in this setting, however this is a perfectly reasonable answer" and then explain why the idea could work (i.e. intuitions, sometimes even explaining similar problems where it does work), and finally I would demonstrate why our current task is different and why that particular method fails.

There are particular personalities such that making them ask even a silly question helps. I remember one session where I made the whole class ask clownish things which, then, greatly eased the tension (it transformed into a sort of joke-questions contest) and activated even the most stubborn part of the group (and we did lose a lot of time, but I wouldn't say it was wasted).

After a two-three classes it becomes a standard thing and I don't even need to pick them. If someone is behind, it is often enough to apply a very gentle pressure (like a slightly longer eye-contact) to activate him/her. During the semester (i.e. when I earned enough respect) I sometimes taunt/tease them, even to the point of encouraging disobedience (tricking them into independent thoughts), e.g. proposing some bad solutions (sometimes blatantly bad, e.g. for comical effect, sometimes subtly wrong, to guard them against bad intuitions) or saying "no" to perfectly valid inferences (e.g. to emphasize which part of the process was the key part "oh, now I understand", and perhaps later to recall some previously solved problems "it is the same we did last week" and extract similar patterns, perhaps alike in some unobvious way; or to make the solution more formal and show them some traps which could be overlooked and later lead to incorrect reasoning).

As with any approach, there are disadvantages. There was one group which learned that saying "I don't know" is ok too fast and I was basically helpless - the contact was poor, and as things I prepare depend on class participation, they learned less and less, and that, in turn, made them even more remote. I was unable to break from the evil cycle until the very end.

Another thing is that these people start asking many (too many) questions in classes taught by others which makes them (the teachers) angry at you (e.g. it disrupts their flow).

Also, it works better with mature people, i.a. ones which are conscious that it is in their interest to learn, rather than just earn grades. One time such a situation turned itself into really dissonant tones: I scorned a "let-me-show-off" question of a good grade-earner and praised a bit naive, but honest question from person who usually doesn't know what's happening. There were some resentful looks and I had to give the class a much harder challenge so that it would be difficult even for the top part, so that the aforementioned person could reclaim his status, but in the process had to ask honest questions to complete the task.

Perhaps this is not the best way, but it works for me (among others, it matches my style of teaching).

I hope this helps $\ddot\smile$