The best way to teach preschoolers is to use a hands on curriculum. In the science field that can get messy. Autumn leaves are colorful, but fragile and make it hard to view them first hand in the classroom. I use Discovery Bottles to keep them from crumbling and yet, allow students to get a close up view of the leaves.
I added a few glittery leaves to the bottles to make them more intriguing.. Magnifying glasses are nearby for an even closer look.