Model and explain the greenhouse effect: how certain gases let sunlight in but trap outgoing heat, warming Earth's surface.
Sunlight passes through Earth's atmosphere fairly easily and warms the surface below. That warmed surface then radiates heat back upward, but certain gases in the atmosphere, mainly carbon dioxide, methane, and water vapor, called greenhouse gases, absorb much of that outgoing heat and re-radiate it in all directions, including back down toward the surface. This natural process, the greenhouse effect, keeps Earth's average temperature far warmer than it would be with no atmosphere at all, and without it, Earth would be too cold for the life it currently supports.
Human activities, especially burning fossil fuels like coal, oil, and natural gas, have added a substantial amount of extra carbon dioxide to the atmosphere over the past two centuries, which strengthens the same natural greenhouse effect. You can model this with a simple test: two identical closed containers in sunlight, one with extra carbon dioxide added, one with ordinary air, both fitted with a thermometer. If the added-carbon-dioxide container consistently reads a higher temperature after time in the sun, that is direct evidence of the greenhouse effect in miniature.
Mastery looks like: They can explain the greenhouse effect mechanism, sunlight in, heat trapped, and can describe a fair test that models it.
Common stumbles: Thinking greenhouse gases block sunlight from entering, which they do not; assuming the natural greenhouse effect itself is the problem rather than the added amount of gas.