How do monarch butterflies travel so far?

from  Brains On! Science podcast for kids

Monarch butterflies are unique — they’re the only butterfly to travel thousands of miles when the seasons change. They travel from as far north as Canada all the way down to a few very specific mountaintops in central Mexico. They don’t have a car, or an airplane ticket. They just have their two little wings. So we’re asking: How do they do it? How do they migrate thousands of miles? And why?