Always wear eye protection during science experiments. Never handle sharp objects, open flames, chemicals, or other dangerous objects without adult permission and supervision.
Fill each of the drinking glasses to about the halfway point, one with hot water and the other with cold water. Alternatively, if you like, you can try this experiment with more glasses and a range of different temperatures.
Put a single drop of food coloring in each glass, trying not to give any of them a head start (you can try using both hands or asking a labmate for help). And remember, don't stir!
Carefully observe how the food coloring spreads throughout the water. Which one is spreading the quickest?