You may have heard that monarch butterflies migrate from Mexico to Canada and back.

But you may be more amazed to know that the returning butterflies are the great-great-grandchildren of the butterflies that left.

Here’s how that works:

The first generation of monarchs leaves Mexico in the spring, pausing to breed and lay eggs as they fly northward.

Their eggs hatch into caterpillars, which eat for two weeks, then metamorphose into butterflies. In four weeks, they too are flying northward.

As the first generation dies, the second generation will fly on, farther north, pausing only to lay eggs of their own. Eventually they too will die, and be passed over by their offspring.

This third generation will finally reach Canada, where they’ll lay eggs.

But the fourth generation that comes from them will be genetically different.

The shorter days and colder temperatures cause these butterflies to develop much larger fat stores, making up a third of their bodies.

And their reproductive organs remain undeveloped—since their purpose is not to breed, but to fly.

Before winter arrives, they’ll make the entire 3,000-mile journey to Mexico—where they’ll seek out the same mountaintops as their great-great-grandparents.

They’ll roost in the same fir trees, congregating in huge masses to hibernate through winter.

In spring, their reproductive organs develop, and they’ll begin their own flying, mating, egg-laying journey, to start the amazing monarch relay, all over again.