Open by naming the four big topics the class will explore this term: living things and their environment, reversible and irreversible change, sudden changes to Earth's surface, and electrical circuits. Tell students that across all four, they will work the way scientists do — and that is what today is about.
Introduce the three jobs a variable can have in an investigation. Model with one concrete scenario (a toy car rolling down a ramp) and think aloud:
Demonstrate the ramp once at a low height and once at a high height. Think aloud: "I changed only the height. I kept the car and the floor the same. That's what makes it fair — so if the car rolls further, I know it was the height that did it."
Then model a prediction as a reasoned guess, not a wild one: "I predict the higher ramp will send the car further, because a higher start gives it more energy to move." Underline that a good prediction says what and why.
Work through two fresh scenarios together, with students using mini-whiteboards so everyone commits to an answer.
Scenario A — "Which paper towel soaks up the most water?" Ask the class to tell you, for this investigation:
- What is the one thing we will change? (the brand of paper towel)
- What will we measure? (how much water it soaks up)
- What must we keep the same? (amount of water, size of towel, time left in the water)
Take whiteboard answers, name the variables on the board using the change / measure / keep-the-same headings, then ask one student to phrase a reasoned prediction aloud.
Scenario B — "Does a plant grow taller in more light?" Run the same routine, but this time deliberately offer a flawed plan ("let's also give the sunny plant more water") and ask: "Is this still a fair test? Why not?" Use their answers to reinforce that changing two things at once spoils the result.
Students work independently on the investigation-planning template for a new scenario:
"Which type of ball bounces the highest?"
On the template, each student writes:
- the variable they will change;
- what they will measure, and how;
- at least two variables they will keep the same;
- a prediction that says what they think will happen and why.
Circulate and check that each plan changes only one variable. Pull a small group for support if the change/measure distinction is still shaky.
Exit ticket (last 3 minutes): give students this scenario — "Testing whether warm water or cold water dissolves sugar faster" — and have them write on a sticky note one variable to change and one to keep the same.