A living entity is referred to as an organism. In order of size: cell, tissue,organ,organism.
So every organism is individually classified without the worry of mixing up two organisms like a fly and nat.
The answer to your question would be space. The sun's light travles through space. but if you clap, you can't hear it.
By examining the tree bark and recording it’s properties. I would note if the cells move and duplicate, I would also note any living organisms which lay on the surface
