The Moldy Cups and the Conclusion That Water Is Bad
Ms. Barrett's Grade 2 class (21 students) is three weeks into a bean-seed investigation. Each group planted four beans in a cup with potting soil. Group A keeps cups on the windowsill and adds 30 mL every other day. Group B keeps cups in a cupboard and adds 30 mL daily. Group C keeps cups on the windowsill, uses cups without drainage holes, and adds 60 mL daily. On Monday, Group B's cups have pale, spindly shoots and a sour smell; Group C's have fuzzy green mold and no sprouts; Group A's have healthy 6-cm seedlings. During share-out, students conclude that water is bad for plants and that mold is a kind of plant. Jonah adds that plants eat dirt, and several classmates nod. A parent volunteer with a mold allergy is scheduled for Wednesday. The class has one sink, no fume hood, and a custodian who handles biohazard bags on request.
1. How do you handle the moldy cups and the class's conclusion without turning the investigation into a teacher lecture?
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Safety first. Do not open the moldy cups. Have the custodian double-bag and remove them, wash the table and hands, ventilate the room, and move the volunteer with a mold allergy to another station. Photograph the cups before disposal, inside a clear bag, so the evidence is preserved without exposure.
Then treat the mold as data, not as a failed experiment. Ask students to compare the three sets of conditions: drainage, amount of water, and light. Group A had drainage, 30 mL every other day, and light. Group C had no drainage and 60 mL daily. Group B had daily water in a dark cupboard. The mold and rot pattern is evidence about excess moisture and poor drainage, not about water being harmful. Rebuild the claim: Too much water and no drainage harmed the plants, rather than water is bad.
Mold is not a plant. Use a hand lens on a sealed bag and a simple image sort: mold is a fungus; it does not make its own food and does not have leaves. This is a classification point, not a detour.
Do not quietly discard the cups. Silent removal teaches that unexpected results are hidden. A claim-evidence-reasoning chart lets students revise their thinking publicly, which is the scientific practice the standard actually names.
2. Redesign the investigation to test one variable, and explain how you would address Jonah's plants-eat-dirt idea with evidence.
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Redesign with one variable. Test water amount only: 10 mL, 30 mL, and 60 mL every other day, with drainage holes, same seed type, same soil, same planting depth, same window, same cup size. Keep a watering log and measure days to sprout, height twice weekly, and number of leaves. If light is the question, run a separate parallel test with the same water regime.
Address plants eat dirt with evidence students can see. Ask: what would we observe if plants ate soil? Soil level would drop noticeably, and plants grown without soil could not grow. Set up a small comparison: one bean in soil, one in water with dissolved plant food, both in light. The soil level barely changes while the plant gains size, which challenges the eating-dirt model. Introduce the accurate idea concretely: soil holds roots and supplies water and minerals; the plant makes its food using light. Do not demand the full photosynthesis model in Grade 2; aim for plants need light, water, air, and minerals.
Keep the investigation fair. Same seed, same cup, same soil, same amount of light, one difference. Students should be able to say what was changed, what was kept the same, and what was measured. The moldy cups become a lesson in why drainage and amount of water must be controlled, not a reason to abandon inquiry.
3. What assessment evidence would convince you that students understand a fair test and the needs of plants, rather than just remembering the words?
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Assess the practice, not the vocabulary. Use three evidence sources.
1. Transfer task. Give a short scenario: two students test whether light matters, but one waters daily and one waters weekly. Students identify the problem and revise the plan. A student who says they changed two things, so you cannot tell which mattered is demonstrating the fair-test concept.
2. Data claim. Students write or dictate: Our plants grew tallest with ___. I think this because ___. Score whether the claim matches the data table, not whether the sentence is long.
3. Sort and explain. Provide six cards (mold, bean plant, sunlight, soil, water, sprout) and ask students to sort needs versus organisms, then explain one card. This catches the mold-as-plant misconception and the plants-eat-dirt idea without a recall quiz.
Record results on a simple 3-point rubric: identifies the variable, uses data as evidence, revises a claim when evidence conflicts. Keep the moldy-cup episode in the portfolio as a formative example; students who revised their claim should get credit for the revision, which is the scientific practice the standard asks for.