Influential work on human thinking suggests that our judgment is often biased because we minimize cognitive effort and intuitively substitute hard questions by easier ones. Recent work with adults who solved the bat-and-ball problem, one of the most publicized examples of the substitution bias, suggests that people realize they are doing this and notice their mistake. In the present paper we look at the development of this substitution bias sensitivity. A group of young adolescents solved standard and isomorphic control versions of the bat-and-ball problem in which reasoners experience no intuitive pull to substitute. Adults have been shown to be less confident in their substituted, erroneous bat-and-ball answer than in their answer on the control version that does not give rise to the substitution. However, the present study established that this critical confidence drop was less pronounced for young adolescents. This implies that in contrast with adults, young adolescents do not yet fully acknowledge the questionable nature of their biased answer and remain more oblivious to the substitution. That is, young adolescent reasoners seem to behave more like happy fools who blindly answer erroneous questions without realizing it.