Prop a tray against a cushion to make a ramp and gather a mix of objects: a ball, a toy car, a block, a spoon.
Let your child send each one down and watch what happens. Say what you both see: 'the ball rolled, the block slid.'
Then raise the ramp higher and try the same objects again. Ask what changed before you say anything yourself.
Varying one thing — the slope — while keeping the objects the same is the shape of a controlled comparison. At this age the child is not reasoning about it explicitly, but they are collecting the experience that later makes the reasoning possible.