Write a letter to Grandma and tell her what you've learned about Galileo, introducing the concepts of acceleration and inertia. State that he was familiar with forces but didn't see their connection to acceleration and mass. Tell her how Isaac Newton did see the connection and how it explains why heavy and light objects in free fall gain the same speed in the same time. In this letter, it's okay to use an equation or two, as long as you make it clear to Grandma that an equation is a shorthand notation of ideas you've explained.