1. They attach themselves to children's hair, shoes and clothing.
2. They attach themselves to pets.
3. Family members hold the front door open for extended stretches on windy days as part of your "Let's Heat the Whole Outdoors" program.
4. Art projects.
5. Your maple tree is equipped with the latest stealth technology.
6. Some leaves decide they would like to see what it's like being indoor foliage.
7. It's all part of some "Invasion of the Body Snatchers" deal.
8. Family members secretly invite them in because it is entertaining to hear a certain someone say "As soon as I get done sweeping up leaves in here, more appear!"
9. Once inside, they breed.