393. By the Stream. There is a general opinion that puzzles requiring the measurement of a certain amount of liquid can only be solved by a series of trials, but in such cases, general formulas for solutions can be found. Taking advantage of an unexpected leisure, I examined this question more closely. As a result, some very interesting things were discovered. Let's consider, for example, the simplest case, where a person comes to a stream with only two vessels and wants to measure a certain amount of water. If we are dealing with, say, a barrel of wine, we may encounter various complications related to whether the barrel is empty or full, whether we know its capacity and contents, whether the loss of wine is allowed, and whether it is possible to pour the wine back into the barrel. By the stream, all these complications disappear. Perhaps the problem has simplified to such an extent that it makes no sense to talk about it as a puzzle at all? Let's see.

A person comes to a stream with two vessels of 15 and 16 liters capacity. How can he measure exactly 8 liters of water in the fewest number of operations? Filling a vessel, emptying it, or pouring water from one vessel to another counts as one operation.

This puzzle is not difficult, but I think the reader will find it quite entertaining and instructive. It goes without saying that no tricks, such as marks on the vessels or tilting them, are allowed.