Make your objects conform to the laws of physics. For example, they will fall due to the force of gravity. They can also collide with other Physics objects or Wall Objects.
Wall objects will not move when bumped into and cannot be affected by external forces. Wall objects typically have collision boundaries that are used for buildings and the ground.
Scenery objects have no collision boundaries and exist only as a graphic in your scene.
Tip: If you notice that your creation is becoming slow or unresponsive, try making the objects that do not interact with anything else in your creation a Scenery Object.