Question

Why do mustard seeds pop while cooking?

Answer 1

Mustard seeds contain water. When you drop it into hot oil and if the temperature of the oil is more than 100 degrees centigrade, it causes the water induce the seeds to evaporate. This increases the pressure inside the seed and it pops open. Oil has a high boiling point ( not exactly boiling point, but for the current situation let's say so) and normally cooking happens well above the boiling point of water.