Any logical problem involving the measurement of a specified volume of liquid using jars of different capacity, first introduced into cognitive psychology in 1942 by the US psychologist Abraham S(amuel) Luchins (1914–2005) to study Einstellung effects in problem solving. A typical complex water-jar problem requiring an indirect solution is as follows. Given empty jars holding 21 quarts, 127 quarts, and 3 quarts, obtain 100 quarts of water; this is solved by filling the 127-quart jar and then from it filling the 21-quart jar once and the 3-quart jar twice, leaving 100 quarts as required in the 127-quart jar. A simple water-jar problem such as the following can be solved directly. Given empty jars holding 15 quarts, 39 quarts, and 3 quarts, obtain 18 quarts of water; this can be solved directly by filling the 15-quart and 3-quart jars and then emptying their contents into the 39-quart jar. In an experiment with college students, Luchins found that after solving several complex problems, 81 per cent of the students used the same indirect procedure with simple problems that could be solved directly, and only 17 per cent used the direct approach, whereas 100 per cent of students who had not been exposed to the complex problems solved the simple problems directly. Previous success with the indirect technique apparently created an Einstellung or set (2) that blinded students to the direct approach. See also Aufgabe, General Problem Solver, set (2).