The ability to reason about the mental states of others is central to our lives. It is necessary to make sense of human behavior and to communicate effectively. Our assessments of what others know and think also heavily influence our decision-making and our ability to learn from others about our social and physical worlds. Yet, both children and adults exhibit some remarkable limitations in reasoning about what others will know and believe. Previous research has identified a cognitive bias in which our current knowledge state interferes with our ability to accurately remember what we once knew or thought, and to judge what others are likely to know. This talk will discuss the implications of this bias, pointing out an asymmetry in perspective-taking, and highlighting factors that may contribute to this bias such as source monitoring processes and inhibitory control.