- 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
- Therefore, 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
- Sinking means moving down
- Hence, gravity is the force that causes the marble to sink

Hence, gravity is the force that causes the marble to sink
Which force causes a marble to sink to the bottom of a glass of water?
- Sinking means moving down 
- 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