Tina asks the question "Does caffeine increase the heart rate of an earthworm?" In Test 1, she measures the heart rate by looking at the earthworm under a microscopes, the earthworm has a heart rate of 50 bpm (beats per minute). In Test 2, she places a few drops of caffeine on the earthworm's skin and measures the rate again. In this test, the heart rate is 68 bpm. The independent variable is