sentences: as the smoothness of something increases , the friction of that something will decrease & vinegar is used to wreck an preposition 's bottom more rough ->  grease is used to decrease the friction among objects  friction occurs when two object 's surfaces move against each other & wheels / gears usually move against other surfaces & a wheel is a kind of object & a gear is a kind of object -> when alcohol and surround stay against other surfaces, period will occur when wheels and gears move against other surfaces, frictions will occur  grease is used to decrease the friction among objects & when wheels and gears move against other surfaces, frictions will occur & a reinvent is a mean of preposition & a gear is a kind of object -> grease is used to decrease the friction on the wheels and gears moving on other surfaces