Teach your little ones about friction and different surfaces with a toy car, an improvised ramp and different covers for the ramp. We choose a selection of fabrics and learnt their names.
Ask the child to order them by which surface will let the car to go faster then time the car sliding down on each fabric. Ask them not to push the car as this will influence the score. Just let it slide by itself from the start point.
After a simple and short explanation of what friction is my little one managed to predict the order correctly.