Answer:

• Is the tube of tennis balls an object?
  • Yes. It has identity (my tube of balls is different than yours), it has state (opened, unopened, brand name, location), and behavior (although not much).
• Is each tennis ball an object?
  • Yes. It is OK for objects to be part of other objects. Although each ball has nearly the same state and behavior as the others, each has its own identity.
• Could the top two balls be considered a single object?
  • Not ordinarily. Each has its own identity independent of the other. If they were joined together with a stick you might consider them as one object.
• Is the color of the balls an object?
  • No. It is a property of each ball.
• Is your understanding of tennis balls an object?
  • Probably not, although it is unclear what it is. Perhaps it is a property of the object called "your brain."