Glass breaks due to thermal stress caused by uneven expansion. When boiling water is poured in, the inner surface of the glass heats up and expands rapidly. However, the outer surface remains cooler and expands much more slowly or not at all. This difference in expansion rates creates internal stresses within the glass material. If this stress exceeds the glass's strength, it cracks. The correct concept is:

$\sigma \propto E·\alpha ·\mathrm{\Delta T}$ (Stress ∝ Modulus × Coefficient of expansion × Temp change)

Final Answer: The inside of the glass expands faster than the outside of the glass, causing the glass to break.