A strategic puzzle centered around pouring liquids between various bottles, with the goal of measuring exactly n liters.
Decanting problems (aka jug-pouring problems or water-pouring problems) come in a wide variety of forms. One is usually given various bottles (containers) with various known capacities for keeping various liquids. The goal is to design a plan for pouring the liquid between the bottles, so that a certain goal situation is reached.
Example: A classic puzzle starts with two unmarked bottles that can hold 3 liters and 5 liters respectively, and a bathtub with unlimited water. How can 1 liter be measured?