Suppose the question is: “Does the surface affect how far a toy car travels?” In this invented example, car A travels 80 cm on a smooth floor and car B travels 30 cm on carpet. A child concludes, “The smooth surface made it go further.”
The difficulty is that both the car and the surface changed. The results do not show whether the surface alone explains the difference. A better first answer is: “We need to use the same car on both surfaces.”
To improve the comparison, keep the same car, ramp height and release point. Release it without a push, change the flat surface beyond the ramp, and measure the distance from the ramp’s end to where the car stops. Repeating the runs gives more observations to compare.
For practice, use these invented repeat results: smooth surface: 78, 80, 82 cm; carpet: 28, 30, 32 cm. A suitable conclusion is: “In these runs, the car went further on the smooth surface. Every smooth-surface distance was greater than every carpet distance.” That describes this setup without claiming that every car will travel the same distances.
For lower KS2, help the child identify what must stay the same. For upper KS2, ask them to explain why. A follow-up is to propose raising the ramp for only the carpet runs: can the child spot why that would spoil the comparison?