Z's school had a "science fair". Students perform an experiment at home, and present their findings at the fair.
He had a truck roll down a ramp. Steeper inclines made the truck roll longer distances.
Making the truck heavier, with a roll of quarters, also made it roll longer.
We even had one unanticipated result: when we did the experiment on carpet, the heavier truck actually rolled less. I suppose the surface of the carpet resists heavier trucks more than it does lighter ones.
At first, I was sceptical of the idea of a science fair, but it ended up being a good exercise in the process of experimenting. As the lab-director, I insisted that my scientist repeat each experiment three times, and use only the median result, to leave out errors, e.g. a shaken hand, or an incorrect reading. After over 100 separate observations, Galileo needed some chocolate milk.