This cycle is very important. Without the carbon dioxide cycle, there would be no life on earth. We could not breathe. We would have no food. And without limestone, earth would be too hot for us to live. Instead, we have a nice climate. Animals and plants make the carbon dioxide cycle work.

Ocotillo plants in Arizona prefer to grow on limestone soils. Each year in the springtime, the ocotillos reach their red blossoms toward the sky. Hummingbirds come and pollinate the ocotillos. This helps these plants make seeds. The hummingbirds get to drink the sweet nectar of the flowers. But there's more to the story…

Ocotillos and all other plants make oxygen. Without this work, hummingbirds would not have any oxygen to breathe. Neither would we. Animals and people inhale oxygen. Then, animals and people exhale carbon dioxide. Plants use the carbon dioxide to build their stems, roots, and leaves. If the plants had no carbon dioxide, they would die. But there's more to the story…

Volcanoes also give off carbon dioxide. Between the animals and the volcanoes, a large amount of carbon dioxide forms. Fortunately, most of this carbon dioxide turns into limestone. If it did not, all the carbon dioxide would be in our air. This would make the earth very hot. Carbon dioxide traps heat. But limestone locks up harmful carbon dioxide. This makes our world cool enough to live in. But there's more to the story…

Limestone is made from two chemicals, carbon dioxide and calcium. Carbon dioxide comes from the air. Calcium comes from rocks such as lava. When it rains, both chemicals dissolve in the rainwater. Then streams and rivers carry the chemicals to the sea. Clams and snails make their shells from the carbon dioxide and calcium. When these animals die, their shells turn into limestone. If it weren't for clams and snails, there would be hardly any limestone. People would roast because there would be too much carbon dioxide in the air. Too much heat would be trapped.

After millions of years, limestone can rise to the earth's surface. This gives the ocotillo a place to grow. But now you know that there is much, much more to the story.

Vocabulary:

Calcium: a chemical element that occurs in limestone as well as bones and teeth.
Carbon Dioxide: a gas in the air we breathe.
Cycle: anything that happens again and again in the same way.
Exhale: breathe out.
Inhale: breathe in.
Limestone: a sedimentary rock, usually gray in color.
Nectar: sugary syrup found in flowers
Ocotillo: a desert plant that has many tall, spiny sticks and red flowers.
Oxygen: a gas in the air we breathe. All animals must have oxygen to live.
Pollinate: spread pollen from plant to plant to help the plant grow seeds.

Some questions for the students:

1. Why is the carbon dioxide cycle important?
2. How does oxygen get into our air?
3. Why isn't all of earth's carbon dioxide in the air? Where is most of our carbon dioxide?
4. What creatures make limestone?
5. All the animals, plants and rocks on earth are linked together like a chain. What do you think would happen if all the plants died? What would happen if there were no clams or snails?
6. Take the part of any plant or animal in this story. Tell your job in the story and tell how others help you do your job.

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