Report on a nice bit of cognitivist research - as the title suggests, delayed feedback (ie don't give the answer right away) assists retention, and is best done after an unpredictable delay of a few seconds. What's most interesting about it is the hypothesized reason: it's curiosity. It only works if it piques your interest enough to want to know the answer, and your level of attention is raised when the timing of an upcoming anticipated event is uncertain. Like so many things in learning, motivation plays a big role here.