A root cause is something you can change that blocks a whole family of slips.

When you pick which 'why' answer to stop at, test it with two questions: can the team change it, and would changing it prevent similar incidents? Causes that live in a person's momentary slip or in vague pressure are hard to act on. A missing step or check in the process can be fixed once and protects everyone.

Read it once, then a question tests whether the trick stuck.