[Question] What is the top concept that all EAs should understand?

If you had the power to ensure all EAs understood 1 concept back to front. What would it be?

Only one concept per answer.

Please do not read this and feel obliged to understand all these. I don’t want obligation.

I am asking because I might create a test to see how well EAs do know these concepts then see if I can get content for concepts people don’t know. This is meant to make things easier not harder.