Ms. Nelson: Every fall, millions of monarch butterflies fly south across North America. Many of them pass right through Texas on their way to the mountains of central Mexico, where they spend the winter resting together in the forests. Some of them travel more than two thousand miles.

Ms. Nelson: Here's the amazing part: no single butterfly makes the whole round trip. The monarchs that fly south in the fall live for several months. But in the spring, as they head north again, they lay eggs and then die. Their children and grandchildren finish the journey.

Ms. Nelson: Monarch caterpillars can eat only one kind of plant, and that plant is milkweed. A monarch lays its eggs only on milkweed, so without it, there are no new butterflies. In many places, milkweed has been cleared away to make room for roads and buildings. That is why, next week, our class is going to plant a milkweed garden next to the cafeteria.