I asked my students to imagine that we all had a race, and I asked the winner what they had for breakfast. They said they had a banana, so I asked if a scientist could say that whoever had a banana would win a race. They all saw the answer was no, and we talked about why not. This led to the idea of a fair test, which is the basic idea behind the Scientific Method.
Next I had students add the following definitions to their Dropping Spheres Investigation Sheets:
Then I drew a picture of bean plants on a windowsill, and explained a simple familiar experiment, and said I'd water one plant a teaspoon every day, and one plant a 1/4 cup every day, and measure how high they went. I had them talk to their shoulder partners to identify the variable, outcome, and control, and then share with the class. I push them to come up with many controls, even things they may not think make a difference, like the color of the pot.
After reviewing expectations for group work, I have groups send one student to select a sphere from a tub with racket balls, tennis balls, ping pong balls, marbles, etc. I don't put rulers or tape measures out because I want them to start thinking about selecting appropriate tools and helping themselves to what they need.
As they work, I circulate and ask guiding questions such as "What else could you control to keep the test fair?" and "If your group decides to to throw the ball down like that, how would you keep the throws the same each time?"
As groups finish, I have them complete the questions on the end of the investigation sheet together. I allow early finishers to test other spheres as well.
I have groups share back with the class the variable, the control, and the outcome. This is a good opportunity to practice being clear in scientific writing, such as "We dropped the ball from different heights," is much more clear than "how high."
My class did not come up with very many controlled variables, so we talked about all the many things we could keep the same for a fair test, such as who drops and who measures, measuring from the top, middle, or bottom of the ball, and what surface the ball is being dropped on.