In the beginning, an object is just a picture of a thing. Drag a picture of a ball onto your scene and it won't behave like a ball should. It won't bounce. You will have to create events and give your characters Behaviors so that they can come to life. Telling objects to do stuff is fun and exciting - you are creating your own unique world and stories.
To make a ball "smart" it needs to acquire a Behavior. Behaviors are like little bits of DNA that turn an otherwise inanimate object into a creature that has some capability. Once an object has a behavior, it's a smart object.
Recommended Unity Versions
2017.1 and Above
Behaviors Part 1 - Ready Maker
Stay in Frame
You will need to keep your objects in the frame of the screen. This behavior is called "Stay in Frame".
If you don't add this behavior, your game assets will disappear off your screen, and your players won't be able to play your game as it was intended to.