At the start of this lesson, I hand out Petoskey stones. I was fortunate to obtain a class set while visiting Michigan. As I live in Arizona, and most of you also do not live in Michigan, I have provided a document here with links to places to buy Petoskey stones.
I gave the students this information:
This is an animal fossil. It was found in Michigan.
That is all I told them initially. I asked them to closely examine their stones and had them dip them in water to accentuate the fossils' lines. I circulated as they were examining the fossils and asked them prompting questions to extend their thinking.
In this part of the lesson, I ask students to express their ideas about what this fossil might be, and how we could conduct further research. Most of them are far off-base in their ideas, thinking that it must be a snake fossil because it looks like snake scales. This is a perfect because it gives me an easy segue to discussing the importance of not jumping to conclusions. To stick with the snake scale example, I first ask them what part of an animal is usually preserved by the process of fossilization. They are able to answer, "...the bones...", and then from that point, I ask them to re-evaluate their response. Snake skin is very fragile. Additionally, I point out that this description, "hexagonal pattern that looks similar to snake scales" is very specific and helpful, and that they just need to be certain not to go the extra step to saying that the resemblance to snake scale means that it was a snake.
Here are some examples of the types of observations students were making about their Petoskey stones.
Students read this short Petoskey Stone basic reading passage. with a partner and write down 3-5 important details. They then write 3 sentences about how their ideas about the Petoskey stone were correct and incorrect.
Here is an extremely short visual presentation that shows a few of the modern-day colonial corals that are similar to those which formed Petoskey stones.