If you don’t know something in the viva, ask yourself “Why?”
If you feel stuck during your prep, “Why?”
If your supervisor makes a suggestion and you’re not sure about it, ask them “Why?”
Or if your external says they’re not convinced by something in your thesis, ask them “Why?”
(politely, of course!)
The single most helpful question to have on the tip of your tongue, for reflections, for preparation, for meetings, for discussions and in the viva is “Why?” It opens things up, prompts, allows for exploration and probes to the heart of everything. It may not always lead to “the answer” but always generates a response.
Keep it in mind for your submission, preparation and the viva.