Lynn tested the effect of different types of cooking oil on the amount of time it takes for a drop of water to fall through a container of oil. He poured the same amount of canola oil, olive oil, and coconut oil each into three different clear containers. He had his partner start a stopwatch at the exact moment he placed a water drop onto the surface of each oil. He recorded the time it took for each water drop to fall through the oil and analyzed the data. Which variable must Lynn hold constant in order to accurately investigate the effect of different cooking oils on a water drop falling through the oil?