Philosophy
The Metaphysics of Composition: When Do Parts Form a Whole?
Quick fact
Philosopher Peter van Inwagen argues that the only genuinely composite objects are living beings—your body's parts compose a whole only because they work together to keep you alive.
Why this is interesting
You see a pile of sand, then a sandcastle. What makes the first a mere heap but the second a whole? Or is that difference just in your mind?