Step 1: Estimate how long oxygen takes to diffuse across a cell (10 μm) vs a 1 mm tissue slab. D for O₂ in water ≈ 2×10⁻⁹ m²/s. Step 2: Diffusion time t ≈ x²/(2D). Step 3: Cell: x = 1×10⁻⁵ m → t = 1×10⁻¹⁰/(4×10⁻⁹) = 0.025 s. Step 4: Tissue 1 mm: x = 1×10⁻³ → t = 1×10⁻⁶/(4×10⁻⁹) = 250 s ≈ 4 minutes. Step 5: Conclusion: diffusion feeds single cells instantly but starves thick tissue — which is why animals evolved circulatory systems, and why cells stay microscopic. Result: 0.025 s vs ~4 min. The x² law is why biology is built at the micron scale.