While many studies in the theory of mind (ToM) literature have investigated how we understand others' mental states, few have explored the mechanism by which we reflect on our own mental states. This study examined how adults reflect on their own and others' mental states within the same ToM task. To do so, we modified the Smarties task, one of the traditional ToM tasks for children. The results showed that adult participants were biased by outcome knowledge when recalling their false belief and that the participants who overestimated their false belief also overestimated the mental states of a naive other. These results were analogous to young children's failure in the Smarties task. Considering the current findings, we discuss possible cognitive processes that are common across children and adults when reflecting on their own mental states and the mental states of others.