At the North Pole
After the elves have loaded the sleigh with the presents for children around the world, Santa reaches into the pocket of his pants and pulls out a special pouch. Inside this pouch is magical dust made of Christmas dreams throughout the ages. Santa carefully sprinkles each of the reindeer with this dust.
Flying Around the World
If the magical flying dust blows off the reindeer when they're high in the sky, there is another way to keep Santa's reindeer flying. The horns of the reindeer are special receivers of dreams and wishes. The reindeer are able to pick up all the dreams and wishes of children around the world, which makes the opposing forces of lift and thrust work together to keep the sleigh flying.
After Presents are Delivered
Once the presents have been delivered, the reindeer are able to fly because of the sheer joy they and Santa feel, knowing that children all around the world have received their presents. This joy keeps the empty sleigh afloat all the way back to the North Pole.