Joe hung 2 identical metal cups, Y and Z, from a horizontal rod. A lighted candle was then placed directly under cup Y as shown below.
After a few minutes, Joe observed that the rod moved with cup Y moving up and cup Z moving down.
a. Joe explained that cup Y moved up because it had expanded and became lighter. What is wrong with his explanation?
b. Explain what caused cup Y to move up.
Source: Peiying Primary