Carefully thought out juxtaposition can add humor to your public speaking engagements. This entails placing two ideas or objects side by side for comparison purposes. Oxymoron and pleonasm are the two different types of juxtaposition that you can use. An oxymoron occurs when two drastically different concepts or words are placed together. For example, loud silence, jumbo shrimp, and random order. Pleonasm happens when there is redundancy, like young child, positive praise, and killed dead.
You can add a subtle comic value to your public speaking presentation but creating juxtaposition. Use your knowledge of the audience to enhance the impact the words play. There are thousands of websites dedicated to this topic to assist you in your quest to add humor to your public speaking speech (Did ya catch that?)