Previous research has found that young children recognize an adult as being acquainted with an object most readily when the child and adult have previously engaged socially with that object together. In the current study, we tested the hypothesis that such social engagement is so powerful that it can sometimes lead children to overestimate what has been shared. After having shared two objects with an adult in turn, 2-year-old children played with a third object the adult could not see. In three out of four conditions, the adult remained co-present and/or communicated to the child while she played with the third object. Children falsely perceived the adult as being acquainted with the third object when she remained co-present (whether or not she also communicated) but not when she clearly terminated the interaction by disengaging and leaving. These results suggest that when young children are engaged with a co-present person they tend to overestimate the other's knowledge. (Contains 1 table and 3 figures.)