Memory's mind games.

As slide shows go, it wasn't even in the same league as your aunt's vacation snapshots, but the audience was paying close attention: there was going to be a quiz. In one sequence a student sitting in a packed lecture hall topples onto the floor. In others, a hand retrieves oranges that have rolled all over a supermarket, and a woman picks up groceries scattered across a floor. Between 15 minutes and 48 hours later, the Boston University undergrad volunteers in this psychology experiment scrutinize more photos and, for each one, decide whether they ever saw it before. Yup, saw the student carelessly leaning back in his chair. Yeah, also saw the guy stupidly take an orange from the bottom of the pile. Uh-huh, saw the grocery bag rip. In all, 68 percent of the time the students remembered seeing the "cause" picture (a ripping bag) whose effect (spilled groceries) had been part of the show.