Today's lecture is about the migration patterns of monarch butterflies. Every year, millions of these butterflies travel up to 3,000 miles from Canada and the northern United States to central Mexico. Interestingly, the butterflies that make the journey are not the same ones that started the trip; it takes four generations to complete the full cycle. The final generation, which emerges in late summer, has a longer lifespan and is the one that actually reaches Mexico. Scientists believe that they use a combination of the sun's position and the Earth's magnetic field to navigate.