Temperature-controlled chemistry. Each stage produces a different physical result from thread (106°C) through soft ball, hard ball, soft crack, hard crack, to true caramelisation above 160°C where sucrose breaks into hundreds of new compounds. It's a one-way reaction — once caramelised, it cannot be reversed.
A candy thermometer is essential — stages are only degrees apart. Crystallisation is the enemy: don't stir boiling syrup, wash down pan sides, add acid or corn syrup to disrupt crystal formation. The colour tells you the flavour — pale amber is sweet, deep amber is bittersweet, dark brown is seconds from burnt. That transition happens in about 10 seconds. Caramel continues cooking from residual heat.
Light-coloured saucepan and thermometer are non-negotiable. Pull at medium amber — residual heat darkens another shade. Add warm cream in a thin stream. Pinch of flaky salt at end for salted caramel. For dry caramel method: heat sugar directly, faster but less forgiving.
No thermometer. Stirring boiling syrup. Crystals on pan sides seeding the batch. Not having cream/nuts ready — you have seconds to act. Adding cold cream to hot caramel without caution — it erupts. Using a dark pan that hides colour change.