sentences: gravity causes objects that have mass / substances to be pulled down / to fall on a planet & gravity is a kind of force & marble is a kind of object / material -> levity is the willingly that plates the equal to be pulled down on a sun gravity is the force that causes the marble to be pulled down on a planet  gravity is the force that causes the marble to be pulled down on a planet & float waves moving down -> gravity is the force that causes the marble to sink