Topic Versus Focus: Two Different Relevance Tests

Every essay has a topic, the subject it is about, and a focus, the particular claim it is making about that subject. These aren't the same test. Topic is wide: any true statement about volcanoes passes it, including facts about eruption sound, ash color, or myth. Focus is narrow: it only admits sentences that advance the specific argument this essay is making, like how accurately scientists can now predict where lava will flow. A sentence can walk right through the topic gate and still get stopped at the focus gate.

Picture two circles, one inside the other. The outer circle is the topic: everything about volcanoes, or solar panels, or streetlights. The inner circle is the focus: only the sentences that serve the one claim this essay is building, sitting inside one particular paragraph working on one particular point. A sentence has to land inside the inner circle to survive. Being somewhere in the outer ring isn't enough, because the paragraph around it is doing a specific job, and anything that doesn't do that job is weight the paragraph is carrying for nothing.

This is the first move in deciding whether a sentence belongs at all, before any question of grammar or style. Once a sentence clears both circles, the next question is whether it actually supports the claim in its sentence with the right kind of evidence, a number for a number, a measurement for a measurement, not just a related fact nearby.