Design and describe a fair test comparing plant growth in light versus darkness, identifying variables and predicting results based on photosynthesis.
You have learned that photosynthesis needs light, so it is reasonable to predict that a plant kept in the dark should struggle compared to one kept in the light. Testing that prediction fairly requires a controlled experiment: two groups of seedlings, ideally the same type of plant, planted in the same kind of soil, given the same amount of water, on the same schedule, with only ONE thing different between them, the amount of light. The one intentionally different factor is the independent variable, here, light; what you measure as a result is the dependent variable, here, plant height, leaf color, and overall health.
A well-run version of this experiment usually shows a striking result: a plant kept in complete darkness grows unusually tall and thin very quickly at first, called etiolation, as it desperately stretches toward any possible light source, but its leaves stay pale yellow or white, since it cannot make the green pigment chlorophyll or sustain healthy growth without light energy to fuel photosynthesis. Meanwhile, the plant in normal light grows sturdier, shorter at first, and a healthy green.
Mastery looks like: They can identify the independent and dependent variables in this experiment, list the controlled variables, and make a photosynthesis-based prediction.
Common stumbles: Changing more than one variable between the two groups; predicting the dark plant will simply "not grow" instead of describing the more specific etiolation pattern.