How to teach your kid to run a fair test, the heart of every experiment

"Which paper towel is best?" Your kid soaks one with a glass of water and the other with a few drops, then declares the first one the winner. Not quite fair! Learning why that test isn't fair, and how to make it fair, is one of the most important ideas in all of science.

The big idea: change one thing

Scientists call it controlling variables. A fair test changes just one thing and keeps everything else the same. That way, if the result changes, you know what caused it.

In the Next Generation Science Standards, used by many U.S. schools, kids in grades 3 to 5 learn to plan investigations using fair tests in which variables are controlled and the number of trials considered. Even the second-grade plant standard focuses on testing one variable at a time.

Kids can learn this, with direct teaching

In a well-known 1999 study, researchers worked with 87 children aged 7 to 10. Children who got explicit teaching about how to design fair experiments, plus questions about their reasoning, learned the strategy and used it in new situations. Children who only got the questions, without direct teaching, didn't improve. Older children got better at carrying the strategy over to very different problems.

The lesson for parents: don't just hope kids discover fairness on their own. Explain it, show it, then let them try. "To be fair, we have to use the same amount of water on each towel. Why do you think that matters?"

Skill 1: Predict, then test what sinks or floats

1. Write a guess for each object: sink or float?
2. Drop each one in the water.
3. Mark what really happened.
4. Count how many guesses were right.

Good test objects: a coin, a cork, a grape, an apple, a crayon, a plastic spoon, a metal spoon, a ball of foil, a lemon.

Writing the prediction first is key. It's the start of a hypothesis: what you think will happen, and why. Being wrong is great. "I thought the apple would sink because it's heavy, but it floated!" is the start of real learning.

Follow up with a mystery: a ball of foil floats. What happens if you squash it into a tight ball? (Try it!)

Skill 2: Which paper towel absorbs most?

1. Cut same-size squares of each towel.
2. Drip the same amount of water on each, with a measuring spoon.
3. See which one holds the most.
4. Write down the winner.

Spot the fair-test rules in the steps: same size squares, same amount of water. Ask your child: "What else should we keep the same?" (Same kind of water, same way of dripping, same time to soak.)

How do you measure "holds the most"? One way: dip each square in water, let it drip for 10 seconds, then squeeze it into a measuring cup and compare. Kids often invent clever methods. Let them.

The challenge: a fair test, start to finish

The mastery goal is to run a fair test with one variable changed, and explain the result.

1. Ask a question you can test.
2. Change only one thing, and keep the rest the same.
3. Measure or count what happens.
4. Explain what you found, and why.

Testable questions to try:

Do it more than once. The NGSS standard mentions thinking about the number of trials. One throw of a paper airplane could be a fluke. Three throws, averaged, is better data.

Explaining is the final step. "The ball bounced higher on wood because…" Even if the "because" is a guess, putting the result into words is what scientists do.

Coaching tips

• Ask "Is that fair?" often. It quickly becomes their question.
• Let them catch their own unfair tests. A messy first try is a great teacher.
• Keep a science notebook with the question, the prediction, the results and the explanation.

Make it a quest

In Family Quests, this is the Tester skill. Practice quests are predict, then test which items sink or float and try which paper towel absorbs most, and the mastery quest is run a fair test with one variable changed and explain the result. Start free.

Next: kitchen science experiments, or go back to growing plants from seed.