Alright, so why did the water level rise? Now it turns out when we kept the bottle into the container full of hot water, that hot water heated up the glass bottle. And the heated glass then heated the water inside the bottle. Because of which water got heated and it expanded. So once the water expanded, it needed more space to occupy. So we saw that as the rise of the level inside the bottle. Because now the water has expanded so it is occupying more space. So this activity shows how when you provide some heat energy to liquids, even they can expand. And you must have seen this when you are using a clinical thermometer when you have a fever or anything. When you are measuring the temperature of your body, you must have seen the mercury rising. Here the body temperature is really high. So the heat energy is coming from the body itself. This is also an example of thermal expansion or expansion of liquid when you supply some heat energy to it.