How to survive a thesis defense
The thesis defence or viva is like an oral examination in some ways. It is different in many ways, however. The chief difference is that the candidate usually knows more about the syllabus than do the examiners.
Consequently, some questions will be sincere questions: the asker asks because s/he doesn't know and expects that the candidate will be able to rectify this. Students often expect questions to be difficult and attacking, and answer them accordingly. Often the questions will be much simpler than you expect.
In a curious relativistic effect, time expands in the mind of the student. A few seconds pause to reflect before answering seems eminently reasonable to the panel, but to the defender it seems like minutes of mute failure. Take your time.
For the same reason, let them take their time. Let them finish, or even elaborate on, the question.
The phrase "That's a good question" is useful. It flatters the asker and may get him/her onside, or less offside; it gives you time to think; it implies that you have understood the question and assessed it already and that you have probably thought about it before. If absolutely necessary, it can be followed by a bit more stalling "Now the answer to that is not obvious/straightforward..." which has some of the same advantages.
Don't try to bluff your way out of a question. If someone has asked a simple question, and you answer with a torrent of jargon, or refer to some complicated equation, the other observers will probably conclude that you haven't answered a simple question with a simple answer. Now this may be both true and honourable: some simple questions cannot be answered simply. However, if you have to resort to complexity, you could begin by translating the question into your terms, defining them as you go, and, when you think you have answered, at least make an attempt to