There are ways of asking a question that lead to a dead end and others that open everything out. My favourite at the moment is ‘how do I know for certain…?’
This question means the children have to use everything they know about the subject to answer. For example, which of these shapes is a square? A pretty straight forward question. Now try, ‘how do I know for certain this is a square?’ I love this as they have to start their answer with ‘because’, no matter what! Also I emphasize the ‘certain’ and don’t let them stop until they’ve convinced me…for certain!