I do this inquiry lesson in collaborative groups just before I start teaching about significant figures in my Chemistry class. It gives the students a list of numbers with the significant figures underlined and they have to come up with the rules for significant figures themselves. I tried it for the first time this year, and they remembered significant figures much better than in previous years. It only takes 10 to 15 minutes in groups. Then I go over the answers and launch into significant figures. I don't include a key, but the answers are very short, and I assume that if you are teaching significant figures, you will know the answers.