1. Fill a dark-colored can with water. Use a magnifying glass (convex lens) with a diameter of more than 10 cm to focus sunlight on a small bright spot and throw it onto the can to heat the water.
2. Fill a small plastic bowl with water, put a piece of black metal in the water, use a magnifying glass (convex lens) with a diameter greater than 10 cm, focus the sunlight into a small light point and throw it on the metal to heat the water.
3. Fill a small plastic bowl with water, put a piece of black metal or plastic into the water, cover the mouth of the bowl with plastic wrap, and the water can be heated by direct sunlight. (Insulated containers are more efficient)