Viscosity, a liquid's resistance to flow, controls how a droplet stretches before it breaks free. Liquids with higher viscosities form longer necks on their drops than liquids with lower viscosities, because the thicker fluid resists separating. Glycerin stretches the farthest before its drop breaks away, while water and ethyl alcohol barely form a neck at all.

Dissolving sugar in water makes it thicker and slower to move. You drill a small hole in the bottom of a metal cup, then mix five batches of water with 20, 40, 60, 80, and 100 grams of sugar. Pour each batch into the cup and time how long it takes to drain through the hole. As you add more sugar, the drain time rises. The comparison shows directly how each added dose slows the flow.

Warming a liquid makes it flow faster, but the effect is not the same for every liquid. You drill a small hole in the bottom of a metal cup and test water, milk, and corn oil at temperatures from 20°C to 50°C. A stopwatch tracks how long the cup takes to empty through the hole each time. Corn oil starts out flowing very slowly but speeds up sharply with heat. Water flows fastest at every temperature. Comparing the three flow rates shows which liquid responds most to warming.