u/vksdann

If you have sand on the pavement you can slide more easily but if you pick up the same sand and rub it in your arms you can feel it almost peeling your skin off.

My arm is moving nowhere near that. Where does the extra energy come from?

A fire needs oxygen and if you put a cup on top of a candle it "creates a vacuum" by burning all the oxygen (cool trick for kids) so where did the oxygen go? Does it "disappear"?

How come the candle "creates a vacuum" when oxygen is only 21% of the air? Shouldn't the ~80% of air fill the gap created by oxygen?