What are the two things?

This is a version of a revision and summing up game that can be helpful at the start and the end of topics.

The idea is very simple. Sometimes you have a complex topic to revise, communicate or research. It helps to be clear on the fundamentals. At that point you ask "what are the two things that I really need to know". Some people play a three things variant.

What do you think the economics professor said when he was asked the question?