Why does food stick to the pan?

Because proteins form chemical bonds with the metal surface, and because the surface is microscopically rough even when it looks smooth — both of which are manageable once you know what is happening.

The two mechanisms:

Chemical bonding. Proteins contain sulphur and other reactive groups that bond readily with metal atoms, particularly iron. This is why meat, fish and eggs stick and boiled potatoes largely do not.

Mechanical grip. Metal surfaces have microscopic pits and ridges. Food settles into them and locks in place as it sets.

Why heat is the main control. A properly preheated pan does two things: it makes the surface oil less viscous so it flows into the microscopic roughness, and it drives moisture off the food's surface rapidly, creating steam that briefly lifts the food away from the metal — the same principle as the Leidenfrost effect that makes water droplets skate on a very hot pan. A cold or lukewarm pan lets food settle and bond before any of that happens.

The practical rules:

Preheat properly before adding oil, then let the oil heat. The water-droplet test — droplets that skate rather than sizzle away — indicates readiness on stainless steel.

Dry the food. Surface moisture cools the pan and delays searing.

Do not overcrowd. Too much food drops the temperature and releases steam, so the food boils rather than sears — which is exactly when sticking occurs.

Leave it alone. Protein releases naturally when a crust forms. Food that will not lift is not ready, and forcing it tears the surface and leaves it welded to the pan. Waiting another thirty seconds usually solves it.

Room-temperature food sears better than fridge-cold.

Why non-stick works, and why it stops: the coating is chemically inert with very low surface energy, so nothing bonds. It degrades with high heat, metal utensils and abrasion, and cannot be repaired — which is why cast iron and carbon steel are the durable alternative.

The stuck bits are flavour — deglaze them.