Describe an activity to demonstrate that warm air is lighter than cool air.

Step 1. Let's take two paper bags of the same size.

Step 2. Hang both the paper bags to both ends of a stick or rod as shown in figure $(A)$.

Step 3. Tie a piece of thread or rope to the middle of the stick.

Step 4. Hold the stick with one hand to balance it.

Step 5. Place a candle below one of the two bags.

It is observed that the bag below which the candle is placed is used up as shown in figure $(B)$. This is because the air become lighter and expands. This activity proves that warm air is lighter than cool air.