We do a series of "classifying" science experiments in the fall.
Sink and float is by far my favorite. For most of the experiments, I just let the students do it in the science center, but for sink and float, we do it small group style. I like talking about being a scientist. I like emphasizing what good scientists do. "Good scientists ask questions. Good scientists write down things that they learn. Good scientists use the word 'predict' Good scientists wear their safety goggles." Doing this experiment in a small group gives me the opportunity to model good vocabulary. "You made an observation. I observed that the cork floated on top of the water. I think I'll write that down. What do you predict will happen to the rock? Does anybody disagree? Why do you disagree? Give us your idea."
Since I can't post pictures of the students doing sink or float, here is Fergus, who practiced the experiment later in the science center. After our small group lesson, I put little clear bottles with objects in them for the kids to practice some more classifying. It's not as fun as putting it in the water yourself, but for us it was helpful for independent practice.