Why is this the answer?

The answer is lava because this clever riddle uses amazing descriptions to paint a picture of exactly what lava is and how powerful it can be. Let's break down the riddle's two main clues to see how they fit. First, it says "the red tongue of the earth." The inside of the Earth is incredibly hot. When that heat melts solid rock, it turns into a glowing liquid called magma. When that magma bursts out of a volcano, we call it lava. Lava is bright red or orange because it is burning hot, just like fire. As it flows down the side of the volcano, it looks like a huge, thick red tongue that is stretching out from the mountain, which is certainly a part of the earth. The color and the way it moves across the ground match this first part perfectly. The second clue is "that buries cities." Lava flows are unstoppable. They travel slowly but with great force, and they cover everything in their path. A river of lava can spread out and surround whole towns and villages. It does not matter how big the buildings are; the hot, flowing rock will wrap around them, destroy them with its intense heat, and eventually cover them completely. When the lava cools down, it hardens into solid, black rock. Everything that was once there the houses, the streets, the fields is now buried under a thick, new layer of stone. Throughout history, people have had to leave their homes because of this powerful force. Famous cities have been completely covered and hidden by volcanic material. This is why the riddle mentions "buries cities." The riddle is playing on these dramatic and true facts about molten rock. It is a very fun and exciting riddle about one of the most powerful natural occurrences on our planet. It perfectly captures the red, flowing nature and the destructive power of a lava flow coming from the earth.