A monarch butterfly weighs less than a single paperclip. Yet it can fly for months, riding high winds and gliding for hours without flapping. They travel during the day and rest in trees at night, sometimes in huge clusters that bend the branches.
When they reach Mexico in November, the butterflies pile onto the same fir trees, year after year. There are so many that the forest turns orange. When the sun warms them up, they all flutter into the air at once - the sound is like soft rain.
Here is the really amazing part. The butterflies that fly to Mexico have never been there before. Their great-great-grandparents made the trip the year before. Somehow they still know exactly which trees to land on. Scientists are still figuring out how.
The journey back is shared by four generations of butterflies, each one flying part of the way north before laying eggs. The great-great-grandchildren of the ones who arrive in Mexico are the ones who fly back the next year. No single butterfly does the whole trip both ways.