Write a report based on the findings of a fictitious experiment. For example, to prove that birds prefer green bird feeders to other colors, you could devise an experiment with three identical feeders--one green, one blue and one red. Explain that one feeder was left on top of a dog house, one in a dead tree and the other in a small bushy tree. Tell the students that the birds were shown to prefer the green feeder placed in the small bushy tree. Use this or a similar example for your report. Or, consider devising a report that states something evocative, such as "Kids should not watch television" as its unfair conclusion. Hand a copy of the report out to each of the students and read it through with them.
Ask the students what they think about the results of the invented experiment. Speculate that it would be better if everybody painted their bird feeders green so the birds used them more (or that televisions should be taken out of homes with children). Listen to any student who raises an issue with a test. Ask what specifically made the test unfair in order to get them to think about the uncontrolled variables present. In the bird feeder example, birds are hardly likely to sit above a dog house or in a dead tree.
Explain that the issues they raised with the experiment were due to variables. Point out that to determine whether the color of the bird-feeder has anything to do with the amount of seed consumed by the birds, everything else would have to be the same in each test. In this case, the most obvious variable that needs to be controlled is the location of the bird feeder.
Use the location of the bird feeder as an example of a controlled variables This should be managed by the experimenter to assure that the test is as accurate as possible. This leaves the independent variable, the color of the feeder, and the dependant variable, the amount of seed consumed. Present a problem to the students, such as "does expensive bird seed attract more birds than cheaper feed?" and see if they can devise an experiment to test it.
Show children a game that demonstrates the effects of variables such as "Blasto" (fossweb.com). Get them to alter simple factors, such as the angle of the cannon, the weight of Blasto and the amount of powder used to show how these things have an effect on the outcome of the test.