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The independent variable is the heat of the light bulb, the dependent variable is the heat absorbed by the wax substance (or other convective liquid) which makes the substances in the lamp move due to density convection.
I am pretty sure that it is the temperature which is applied to the substance. For example when heat is applied to a solid it melts, causing it to change from one phase to another. Also when a liquid freezes the temperature drops and it converts into a solid.
one controlled variable could be using diffrent hylighters and seeing which one makes the water glow brighter.
Because it makes the apparatus black when heating
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Causation.
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The experiment shows exactly what your title says!
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