Flower nectar

Puzzle. The King of Elves prepared for the party 15 barrels of flower nectar. One hour before the celebration he found out that somebody had poisoned one of the barrels. Fortunately the King had four magical vessels; each of them had a magical property: if one drop of poisoned nectar is poured into the vessel, it changes its colour during one hour at the unknown moment. How did the King find the poisoned barrel using these vessels before the party began? It is thought that adding of drops doesn’t take time.

Solution.

Let’s mark B1-B15 for barrels and V1-V4 for the vessels.

Let’s think that the symbol “1” means we put one drop from this barrel into the vessel, and “0” means we didn’t put any drop from this barrel into the vessel.

Then the King has to do one procedure which can be described with the table:

	V1	V2	V3	V4
B1	0	0	0	1
B2	0	0	1	0
B3	0	0	1	1
B4	0	1	0	0
B5	0	1	0	1
B6	0	1	1	0
B7	0	1	1	1
B8	1	0	0	0
B9	1	0	0	1
B10	1	0	1	0
B11	1	0	1	1
B12	1	1	0	0
B13	1	1	0	1
B14	1	1	1	0
B15	1	1	1	1

So after one hour the King can see the unique reaction which shows him the number of the poisoned barrel. For example, if he has seen that the vessels V2 and V4 have changed their colors then the barrel B5 was poisoned.