In the first group of sample problems, some of the questions were normal problems. Others, though, were trick questions. These trick questions contained elements that hinder the process of coming to the correct solution. An example of such a question is #8, "What do you add to a barrel of water to make it weigh less?" In this case, the effective hindrance is one's mental set. Normally, the only way to make something weigh less is to take part of it away. However, in this problem, you must add something. Naturally, one thinks of something that weighs less than zero, perhaps a helium balloon. The answer is actually "a hole." The holes presence will drain the water from the barrel, thereby making it weigh less. This tendency to react to new problems with a habitual strategy is mental set.

Another hindrance in problem solving is fuctional fixedness functional fixedness is the inability to think of uses for an object other than their usual or intended uses. A very common way of overcomin functional fixedness in a problem is through insight, which unfortunately cannot be controlled.