Egg yolk sets at a lower temperature than the white. So why is it runny?

An egg yolk sets at about 70 °C. Most of the white needs about 80 °C. So why does a soft-boiled egg come out with a firm white and a runny yolk?

Because boiling water cooks from the outside in. The white gets far hotter than the yolk ever does, and the timer just decides how deep the heat has reached. Hold a whole egg at one gentle temperature instead and you get the reverse: a custardy yolk inside a barely set white.

Temperature cooks an egg. Time only decides where.