Sorry I didn't get back to you earlier. Below, swombat intuited what I meant: when you think of things that are white in the entire world, it would likely not

Sorry I didn't get back to you earlier. Below, swombat intuited what I meant: when you think of things that are white in the entire world, it would likely not occur to you to mention a specific white Toyota Corolla five blocks from where you are, or to round off to X million white cars to start the list. The question seems to ask for kinds of things that are white, so answers will tend to be "milk", "snow", "whitecaps"... but even that last raises the question of whether things with "white" in the name or description are valid? If so, then "white X" where X is nearly anything is one good answer, making the question meaningless. In contrast, when thinking of white things in your fridge, you can answer "eggs" if the eggs in your fridge are white, even though eggs may or may not be white in general, so they are arguably not valid for the list of things in the world. I'm not saying you consciously went through this line of reasoning; I'm saying that the question frames different kinds of answers, entirely apart from your mentor's intended lesson of "reducing scope allows you to think more deeply about what's left in scope".