With no one looking, you add 5 mL of a cinnamon solution to a blue balloon, which you tie shut. You also add 5 mL of fresh water to a red balloon, which you also tie shut. You heat the two balloons in...

With no one looking, you add 5 mL of a cinnamon solution to a blue balloon, which you tie shut. You also add 5 mL of fresh water to a red balloon, which you also tie shut. You heat the two balloons in a microwave until they each inflate to about the size of a grapefruit. Your brother then comes along, examines the inflated balloons, and tells you that the blue balloon is the one that contains the cinnamon. How did he know?