1. Begin the lesson by asking students what they know about dinosaurs. Record their responses on the board or on chart paper. The students should be able to create quite a list that includes both accurate facts as well as misconceptions derived from fiction. Save the list to refer to later in the lesson.

2. Pose the question, "How do we know dinosaurs really existed?" Allow the students to share some of their responses. Then show students the Fossilized Dinosaur Bones Flash Image, pausing at each image to discuss how what they are seeing helped scientists learn more about dinosaurs. During the discussion, lead students to the idea that what we know about dinosaurs is based on evidence.

3. Return to the list of what students said they knew about dinosaurs. Go through each item and ask students to think about whether it is fact (there is evidence to support it) or fiction (there is no evidence to support it at this time).

4. Ask each student to quickly share their book with the class, including one interesting fact that they learned from the book. As the students share, ask the class if that fact could be proven using evidence. Continue to reinforce the concept of fact vs. fiction (real vs. made-up). Students should now understand that dinosaurs did in fact exist, and what we know about them is based on evidence.

5. Ask students what evidence we have that dinosaurs no longer roam Earth. Students' responses should center on the fact that no one on the planet has ever seen a live dinosaur. This will lead students to the big question, "What happened to the dinosaurs?"

6. Introduce the word hypothesis. Explain that a hypothesis is a possible explanation of something based on facts. Scientists have been studying dinosaurs for a very long time, looking for evidence to help them figure out why the dinosaurs disappeared. However, because the evidence points to different possible explanations, no one knows for certain what happened. So, different scientists have come up with different hypotheses about what killed the dinosaurs.

7. Project the What Killed the Dinosaurs? Shockwave Interactive on a wall screen. Navigate through the first two pages of the activity, paraphrasing the information. On the third screen is a table showing the different hypotheses scientists have about what caused the Mesozoic extinction and the evidence they use to support them. Click on the "Hypothesis" links on the left side of the table, play each animation (bottom of window), and talk with students about what different scientists think happened to the dinosaurs. Then click on the "Evidence" links at the bottom of the table and discuss each one. Finally, click on the boxes in the table to see how each hypothesis is supported by one or more types of evidence. In closing, emphasize that at this point in time, with the evidence we have, nobody really knows for sure what happened to the dinosaurs.

8. Show the Mammals Get Their Chance QuickTime Video. This resource presents the most popular hypothesis of how mammals were able to succeed the dinosaurs to become a dominant life form on Earth. Discuss with the students the question posed by the video: "What if you turned back the clock?"