Take two similar transparent glasses of bottles having wide mouths. Put a few crystals of potassium permanganate or pour a few drops of ink in the bottle. Fill this bottle with hot water. Fill the other bottle with cold water. Cover the cold water bottle with a thick piece of paper such as a post card. Press the post card firmly with one hand and hold the bottle with the other hand. Invert the bottle and place it on top of the hot water bottle. Hold both the bottles firmly. Ask some other person to pull the post card. Observe what happens. Explain.