A Plant Investigation Where Two Variables Move Together
Mr. Baxter's third graders are investigating whether bean plants need light. Each of four table groups gets one cup. Group A's cup sits on the windowsill and gets 50 mL of water daily. Group B's sits in a dark cabinet and gets 50 mL daily. Group C's sits on the windowsill and gets no water. Group D's sits in the cabinet and gets no water. Students measure stem height with a ruler and record leaf color every few days. After two weeks, Priya says, 'Group C's plant died, so that proves plants need light.' Another student says, 'We should just look it up online instead of waiting three weeks.' The class has eight more cups, potting soil, a sunny windowsill, a cabinet, and three weeks before the science fair, where each group must present a claim supported by evidence from its own investigation.
1. Identify the design strengths and flaws in the current setup, and explain what Group C's result actually supports.
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The strength is that students set up a comparison and chose measurable outcomes (height, leaf color), which is developmentally appropriate for third grade. The flaw is that light and water change together. Group C differs from Group A in water only, so it is a valid comparison for water. Group D differs from Group B in water only too. But there is no pair that isolates light with water held constant at a level where plants survive, so the light question is confounded.
Group C's dead plant is evidence about water: no water, no healthy plant. Priya is attributing the result to the wrong variable, likely because light is the question the class cares about. That is a very common reasoning error at this age and is worth naming explicitly rather than just correcting.
A second flaw is one plant per condition. With n = 1, a single seed that fails to sprout or a cup that gets bumped produces a misleading result, and students cannot see whether a pattern is consistent. A third is an unstated measurement protocol: height from where, measured when, by whom, and what counts as 'green' on the color record. These need to be fixed before the data are worth arguing over.
2. Redesign the investigation within the available materials so that it is a fair test, and specify the data the class should collect.
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Use all eight cups for one question: does light affect bean plant growth? Put four cups on the windowsill and four in the cabinet. Give all eight the same amount of soil, the same 50 mL of water daily, the same seed depth, and the same pot type. Rotate the windowsill cups every other day so no single cup is always in shade, and keep the cabinet closed except at watering.
This builds in replication: four plants per condition means the class can ask whether the pattern holds for most plants rather than betting everything on one cup. If students also want to test water, that becomes a second round after the fair or a clearly separated second question, not a fourth group folded into the same design.
Data to collect every three days: cup ID, condition (light or dark), stem height in centimeters from the soil line to the growing tip, number of leaves, and a leaf color rating on a class-made 1-to-4 scale with a printed color key so ratings are comparable. Record who measured and when. A simple class data table with columns for these variables lets students look across the four replicates, compute a group average, and describe the spread instead of reporting a single number.
3. How should Mr. Baxter respond to the student who wants to look the answer up online, and to Priya's claim?
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Both responses should protect the investigative norm without dismissing the students.
To the online suggestion: distinguish background research from evidence. Looking up how long beans take to sprout, how to measure stem height reliably, or what other classes found is legitimate scientific work and will improve their design. But the science-fair standard asks for a claim supported by their own data, and no website can supply that. A useful line: 'We can read to plan better and to compare later; we cannot read our way out of measuring.' If time allows, let students find one source and then list what their own data can add to it - that turns the impulse into a strength.
To Priya: treat the claim as a hypothesis to check against the design, not as a wrong answer. Ask which two cups she is comparing and what is different between them. She will find two differences, water and light, and the class can then ask which difference explains the dead plant. Then have her restate the claim with the variable her evidence supports: 'Plants need water to stay alive.' Asking her to predict what the light comparison will show, and to commit to that prediction before the new data arrive, keeps her invested in the corrected design.