"The oldest of me alive today were saplings before Christ was born. I clean your air, cool your city, and hold the soil together. I am the tallest living thing on Earth. What am I?"

Answer: A Tree. Some trees, like the bristlecone pines of California, are over 5,000 years old. Trees absorb CO2, produce oxygen, reduce urban temperatures, and prevent soil erosion. The coast redwood can grow over 115 metres — the tallest living organisms on Earth.