Make an Easy-to-Understand Presentation

I am currently [reporting situation and purpose]. My presentation topic is [topic]. Please provide [number] ways to start, which should be simple enough for [target audience] to understand, while also being engaging enough to hold their attention. I am currently taking a presentation course at National Taiwan University, and one of the assignments is to make a presentation that elementary school students can understand. My presentation topic is opportunity cost. Please provide three ways to start, which should be simple enough for elementary school students to understand, while also being engaging enough to hold their attention.