Presenter notes: Open with a question they will all have an opinion on. Why is a fly so hard to swat? Take a few answers, and expect fast and clever. Then say that the real reason is in this lesson and it is about its eyes, not its speed, and move on without settling it. Also promise them the second thing: by the end they will be able to explain why there has never been an insect the size of a dog. Both questions sound like fun and both have serious answers.