A good IA question must have variables you can actually afford to test many times—so aim for a simple independent variable with at least three clear levels, like temperature or concentration. Last month, my daughter's idea to test soil pH against plant growth needed a pH meter we couldn't rent and expensive probes; we had to change it to testing different *types* of available water, because tap, boiled, and rainwater cost us nothing. Sometimes the "best" variable isn't the most scientific one, but the one your family budget can handle for ten repeated trials.
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