Each year, Eastern monarch butterflies set off from Northeastern US and Canada to Mexico and back on a round-trip of almost 10,000km (6,000) miles. But each butterfly only completes part of the journey in their lifetime. In the nine months that it takes to complete the round-trip, five generations of butterflies have been born. So, with no older butterflies to follow, how do the new generations know the way? Scientists think they may use the Sun and the Earth's magnetic fields to navigate the long migration.