The way to use a pan without food sticking is to heat it empty over medium heat for a few minutes before adding any oil. As the metal warms it expands, closing off the microscopic pores on its surface; oil only forms a truly slick layer once it's poured onto an already-hot surface.
Pouring oil into a cold pan and adding food right after is, especially on stainless steel and cast iron, the most common cause of sticking. The order of heating matters just as much as the ingredients in the recipe.
How do you know the pan is hot enough?
The practical way to check whether a stainless steel pan is hot enough is the water-drop test: if a few drops of water flash into vapour and scatter the moment they hit the pan, it isn't hot enough yet. Once the water beads up into small, round droplets that skitter across the surface, the pan has reached the right temperature.
At that point oil should be added and heated for a few seconds; once it starts rippling slightly, the food can go in. Cast iron heats up more slowly, and heating it over medium-low for about 5-6 minutes is usually enough.
Why should oil go into an already hot pan?
Oil poured into a cold pan seeps slowly into the pan's pores as it heats, never quite forming a full barrier between the metal and the food. Oil poured into an already-hot pan spreads into a thin, even film instead, which cuts down on sticking considerably.
The moment just before the oil nears its smoke point is the ideal time to add food; at that stage the oil turns slightly glossy and flows more freely.
What happens if food goes in too early?
When meat, fish or vegetables are placed on a surface that isn't hot enough yet, their proteins physically bond to the metal, and trying to lift them before a crust forms tears them apart. Waiting a couple of minutes before flipping lets that crust release itself from the pan.
Food cooked on a properly heated pan usually loosens and lifts easily on its own; if it's still stuck, that's generally a sign the pan wasn't hot enough.
Does it differ for cast iron and non-stick pans?
A well-seasoned cast iron pan can be nearly as slick as a non-stick one when heated correctly, but it should be warmed up gradually rather than blasted with sudden high heat, or it won't heat evenly. Non-stick pans, meanwhile, usually only need medium heat, since excessive heat can damage the coating.
For both pan types, a short warm-up on medium heat followed by oil gives the safest result, rather than leaving an empty pan on high heat for a long stretch.
Does the type of stove change preheating time?
On gas stoves the flame wraps directly around the base of the pan, so heating tends to be fast and even. On induction stoves, heat is generated directly in the pan's base and often warms up even faster than gas, though the pan needs a magnetic base compatible with induction.
On electric coil stoves, heating is slower and can sometimes be uneven; with these it's worth waiting a couple of extra minutes before running the water-drop test for a more reliable result. Whichever stove is used, the test itself works the same way.