Well, we usually assume that the resistance of a circuit component is constant and doesn't change. But the truth is that for anything that conducts current, its resistance always increases somewhat when it warms up.
For things like light bulbs, electric toasters, space heaters, electric stove burners, the heat coils in a blow-dryer ... anything that's designed to be really hot when it's doing its job ... the resistance of those things increases significantly when they come up to their operating temperatures.
