Here's why:

* Energy Flow: Energy flows through the biosphere in a one-way direction. It enters as sunlight and is captured by producers (plants) through photosynthesis. This energy is then passed on to consumers (animals) who eat the producers.

* Loss of Energy: At each transfer of energy between organisms, some energy is lost as heat. This means that energy is constantly being lost from the biosphere as it flows through the food chain.

* Matter Cycling: Unlike energy, matter (like carbon, nitrogen, water, etc.) is constantly recycled within the biosphere. It flows between organisms and the environment, but doesn't permanently leave the biosphere.

Therefore, energy is the only thing that permanently flows out of the biosphere. Matter is recycled and remains within the system.