I usually ask what's your strongest language; then ask questions about that programming language. f.e. if it is python:

> how would you explain the with statement to a junior developer ? then increasingly difficult questions that go into the language runtime/concepts.

one other favourite question of mine is:

> Imagine, you got a standard website the serves data from a database. When a customer types in the url into the browser bar what needs to happen until the customer see the website.

> Go as deep as you can in the answering the question.

When you got an answer, you'll see frontend engineers explain more about the browser, while backend engineers talk more about the backend.

There was one very senior engineer, that actually talked about the ethernet layer, he talked for more than 15min. Most medior engineers are done in 5mins. ;)

There is so much that can happen between the browser making a request and receiving a response, that someone could completely gloss over the OSI model and still talk for a very long time.

I've toyed around with starting my answer with what happens when the physical return key on the keyboard is pressed. Could spend quite a bit of time on what happens before the browser even knows the return key was pressed :D