To name a thing's sphere, ask what the thing is.

Ask what the thing is, not where it is: the trout lives in the water, but the trout is a living thing.

The four spheres: the land is the geosphere, the water is the hydrosphere, the air is the atmosphere, and the living things are the biosphere.

What is the river? The river is water, so the river belongs to the hydrosphere.

What are the pebbles under that water? The pebbles are rock, so the pebbles belong to the geosphere.

What is the trout? The trout is a living thing, so the trout belongs to the biosphere.

What is the wind over the river? The wind is moving air, so the wind belongs to the atmosphere.