In this study, Grade 9 students wrote experiment manuals for their peers describing a simple physics investigation to explore whether air takes space. Peers executed these manuals and their processes were videotaped. In several experimental conditions, these videotapes were played back for authors. Then they had to rewrite the experiment manual. Three weeks later they wrote a letter-of-advice, explaining to peers how to write an experiment manual. Both measures (rewritten manuals and letter-of-advice) showed clear effects of the condition in which writers saw real-time readers' feedback on their own manual, on understanding of the genre of an experiment manual, as well as on the understanding of physics topics introduced.