You have three empty jars, A, B, and C, and a tap. Each jar holds a set amount. Your job is to end up with exactly the goal amount in one jar. You can fill a jar, empty it, or pour one jar into another until the first is empty or the second is full. In 1942, Abraham Luchins used problems like these to show mental set: once a method works a few times, we keep using it, even when a much simpler one is right in front of us.