"I refuse to cede you the role of instructor by letting you define the hypothetical."

You know, come think of it, that's actually a very good description of the second person... who is, by the way, my dad.

I am a lot more successful if I adopt the stance of "I am thinking about a problem that interests me," and if they express interest, explaining the problem as something I am presenting to myself, rather than to them. Or, if they don't, talking about something else.

This hasn't ever occurred to me, but I'll try it the next time a similar situation arises.