2 studies investigated young children's understanding that as the retention interval increases, so do the chances that one will forget. In Study 1 (24 3-year-olds and 24 4-year-olds), 4-year-olds but not 3-year-olds understood that of 2 characters who simultaneously saw an object, the character who waited longer before attempting to find it would not remember where it was. In study 2 (24 3-year-olds and 24 4-year-olds), 4-year-olds but not 3-year-olds understood that of 2 objects seen by a character, the object that was seen a "long long time ago" would be forgotten and the object seen "a little while ago" would be remembered. The findings are discussed in relation to research on young children's understanding of the acquisition, retention, and retrieval of knowledge over time.