We often think of the world as a collection of objects with discrete boundaries. The objects around us: computers, chairs, pens seem to be individually conceivable and easily separable from other objects. These objects sometimes make up larger objects and/or are comprised of smaller objects. For example, a car's engine is an object that is part of the car itself, but the engine can be broken down into smaller pieces, each of with can be considered independently.

But there are other objects that are harder to understand in this way. For example, the Earth. On first attempt, we may try to define the earth as everything made up by material from the Earth. So would we include orbiting satellites, made by metals mined from the Earth, as part of the earth? And how about the Voyager 1 satellite that is 22.2 billion kilometers away from the earth right now? If an asteroid strikes the planet, does this new matter become part of the earth or does it always stay alien? Are we part of the Earth only while we live on it's surface?

Another example to consider is the sun. Should we consider the light energy emitted by the sun as "part" of the sun? What about the heat it gives to our planet? How much is a plant on earth is really part of the sun? It wouldn't be unreasonable to consider all energy originating from the sun as "part" of it, but this is very different from how we normally partition the sun from other objects.

Finally, consider our own bodies and the food we eat. At what exact point does the food become part of us? Once we have broken it down into the fats and proteins that form cells attached to our existing cells? Once the food enters our mouths or hands? Once we have possession or ownership of it? Would the crops in someone's garden be considered part of the gardener long before they ripen?

Once examined closely enough, we see that the concepts of objects having discrete boundaries is a useful mental model we use, and not a fundamental property of the universe.