What happens when you drop food coloring into hot water versus cold water? The color moves at very different speeds. This is convection (the movement of molecules in a liquid as temperature changes). Fill one heat-proof jar with ice-cold water. Fill a second with room-temperature water and a third with hot water. Add one drop of food coloring to each jar at the same time. Watch how the color spreads in each jar. The difference shows how temperature controls the speed of molecular movement.