The Feynman Explanation

Imagine explaining this to a 12-year-old:

"What makes something alive? Most people say living things grow and reproduce. But here's the problem — crystals grow too (they just accumulate material on their surface, like salt forming in a saturated solution). And lots of living things can't reproduce — mules (born from a horse and donkey) are completely sterile but they eat, breathe, and run.

So we need something that ONLY living things do, and that ALL living things do.

The answer is metabolism — the set of chemical reactions inside every living cell. Every living thing — from bacteria in a hot spring to whales in the ocean — is constantly running thousands of chemical reactions: building molecules, breaking molecules, using energy, making energy. Even a plant sitting still is busy inside doing photosynthesis (building glucose from $CO_{2}$) and respiration (burning glucose for energy).

A crystal? No reactions happening at all. A stone? No reactions. A dead body? The reactions stop — that's actually what death IS.

So metabolism is the one thing that ALL living things do and NO non-living thing does. That's why it's the defining feature."