Law of Surprise

Law of Surprise, a custom as old as humanity itself, is the price a man who saves another can demand. It is a request of which neither the saviour or the saved one knows what it is, untill the man saved returns home.

"the first thing that comes to greet you"
When requesting "the first thing that comes to greet you", some examples of the price are:
 * A dog
 * A halberdier at the gate
 * A mother-in-law "impatient to holler at her son-in-law when he returns home"

"what you find at home yet don't expect"
When requesting "what you find at home yet don't expect", the price could be a lover in the wife's bed, but sometimes, a child.