Gravity and Mass and the role of Gravity in the Solar System
One concept in middle school science that can be tricky to teach is the relationship between gravity and mass and the role of gravity in the solar system.
One way I teach this is with barycentric balls. I created my own using string, sand, and tennis balls.
The barycentric system shows that rotation will happen closer to the ball with a greater mass.
The black tennis ball is filled with sand and it has a greater mass.