Which things have more friction? Try to predict which things will be harder to push, and then measure this and see for yourself whether your predictions are true.
Get a plain wooden board and put it on the ground. Put things on the board and try to push them with different amounts of force. It's hard to measure how much force you are putting in without equipment, but see if you can move the thing by blowing on it, or by pushing it just with one finger, or by pushing with one hand, or with your whole body, or even more than one person pushing. Try pushing a wooden block, a toy car, a scrubbing brush (on the bristles and on the wood side), an ice cube, a block wrapped in sandpaper, a piece of jello or a hard-boiled egg, a brick.
Which things are easier to push? Which things are harder? What happens if you pour water on the board before you put the things on? What if the water freezes into ice? How about oil? Does it matter if the things are smaller, if they are just as heavy? Why?
What happens if you tilt the board? Does that change the amount of friction? Or is some other force involved?
Bibliography and further reading about friction:
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