Then Heidi went on to give him an account of the whole day, and of how delightful it had all been, and particularly described the fire that had burst out everywhere in the evening.

The burst causes neurons in a small patch of the brain to fire, and they in turn send signals to other neurons, making them fire as well.

In everyone's life, at some time, our inner fire goes out. It is then burst into flame by an encounter with another human being.