NECESSARY VS SUFFICIENT

A NECESSARY condition has to hold for X. A SUFFICIENT condition guarantees X. They're often confused — most casually, by people arguing about what 'caused' something. A spark is sufficient for a forest fire given dry brush; the dry brush is necessary but not sufficient on its own.

The practice trains the distinction by forcing you to name both, separately, for cases where one is doing the work.

1. 1.Pick three claims of the form 'X causes Y' or 'X is the reason for Y.'
2. 2.For each, ask: is X necessary for Y? (Could Y happen without X?)
3. 3.Then ask: is X sufficient for Y? (Will X reliably produce Y, even alone?)
4. 4.Most causes are necessary but not sufficient — they need other conditions to actually produce the effect. Note which ones are which in your three examples.
5. 5.Bonus: find a case where someone argued 'X causes Y' but X was neither necessary NOR sufficient. (Common in pop-science writing.)