Worked example: Why does glass soften gradually while ice melts sharply? Use the structure picture. Step 1: Ice is crystalline: every water molecule sits in a hexagonal lattice held by hydrogen bonds of nearly identical strength. Step 2: Heating adds thermal energy; at 0°C the energy everywhere crosses the bond-breaking threshold together — the whole lattice collapses at one temperature (latent heat absorbed at constant T). Step 3: Glass is amorphous silica: Si–O bonds have a wide spread of lengths and angles, hence a spread of bond strengths. Step 4: On heating, the weakest bonds loosen first, then progressively stronger ones — viscosity drops continuously over ~100°C around Tg, with no sharp transition. Conclusion: sharp melting = uniform bonds in a lattice; gradual softening = disordered bonds with a strength distribution — structure explains the thermal behaviour.