Caveat is that this is for questions around your work, and not fundamental knowledge you should have learnt in first year!

I saw my committee visibly all sit back when I said this, and I knew I had passed at that point. All the rest was a fun conversation.

Be confident in saying "I don't know the answer to this, but I think...". I've chatted with profs who delight in leading students down the garden path waiting for this,but watching them squirm as they dig themselves deeper.Students will not want to admit they don't know something,but it's ok!