Older (but not younger) preschoolers understand that knowledge differs between people and across time.

We examined 3- to 5-year-olds' understanding of general knowledge (e.g., knowing that clocks tell time) by investigating whether (1) they recognize that their own general knowledge has changed over time (i.e., they knew less as babies than they know now), and (2) such intraindividual knowledge differences are easier/harder to understand than interindividual differences (i.e., Do preschoolers understand that a baby knows less than they do?). Forty-eight 3- to 5-year-olds answered questions about their current general knowledge ('self-now'), the general knowledge of a 6-month-old ('baby-now'), and their own general knowledge at 6 months ('self-past'). All age groups were significantly above chance on the self-now questions, but only 5-year-olds were significantly above chance on the self-past and baby-now questions. Moreover, children's performance on the baby-now and self-past questions did not differ. Our findings suggest that younger preschoolers do not fully appreciate that their past knowledge differs from their current knowledge, and that others may have less knowledge than they do. We situate these findings within the research on knowledge understanding, more specifically, and cognitive development, more broadly.