This is because there are many harmful substances in the rain. Water vapor in the air condenses into water droplets and gradually develops into thick clouds. When the rising air can't support the clouds, water drops will fall to the ground, which is rain. When water droplets pass through the atmosphere, they will carry a lot of harmful gases and dust. This is because factory chimneys and automobile exhaust emit a lot of harmful gases into the atmosphere every day, such as sulfur dioxide, nitrogen oxides, carbon compounds and other harmful gases mixed in the atmosphere. Also, when there is a strong wind, a large amount of floating soil on the ground is brought into the atmosphere by the strong wind, which also seriously pollutes the atmosphere. Rain is like a cleaner. Every time it rains, a lot of harmful dirty things are removed from the atmosphere, so the air is fresh and the sky is particularly blue after the rain. But these rainwater are also mixed with a lot of harmful gases and dust, so they can't be drunk.