The Classroom Plant Experiment
Read this short passage: "Ms. Rivera's class planted four bean seeds in clear plastic cups. Each cup had the same amount of soil and water. Cup A sat on the windowsill. Cup B sat in a dark closet. Cup C sat on a shelf but was covered with a cardboard box. Cup D sat on the windowsill, but its soil was kept very dry. After two weeks, only Cup A had a tall green plant. The class concluded that bean plants need light and water to grow well." A student asks, "Why did Cup C not grow if it was on a shelf?" Another says, "Cup D had light, so light must not matter." The teacher asks the class to identify which cup is the control for the light variable and which is the control for the water variable. They must also explain what the results show about Cup D.
1. Which cup is the best control for testing the effect of light? Explain.
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Cup A is the best control for the light variable because it has light, water, and soil-the normal conditions. Cup B and Cup C both lack light. To test whether light matters, compare Cup A with Cup B or Cup C. Cup A shows what happens with light; the dark cups show what happens without it. Cup D is not a good control for light because it also lacks water, so two variables are changed.
2. A student says, "Cup D had light, so light must not matter." Use the passage to explain the flaw.
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Cup D had light, but its soil was kept very dry. It did not grow because it lacked water, not because light does not matter. The experiment compared Cup A with Cup B and Cup C to test light, and those cups had water but different light. Cup D is useful for showing that water matters, but it cannot be used to decide whether light matters because it changes the water condition.
3. What is the main idea of the passage?
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The main idea is that bean plants need both light and water to grow well. The class tested different conditions: one cup had light and water, some cups had water but no light, and one cup had light but no water. Only the cup with both light and water grew into a tall green plant.