1. A method comprising: one or more processors monitoring instant messages sent by viewers of a video while the video is perceivable to the viewers; filtering the monitored instant messages to identify instant messages having message content relating to the video; estimating a duration of time that viewers are typically expected to discuss, through instant messaging, an event of interest that occurs in the video while perceiving the video; determining a video category corresponding to the video; based on the determined video category, anticipating an event of interest within the video; determining an amount of time associated with the anticipated event of interest; accessing a timeline of the video; selecting a particular point in the timeline of the video; determining a window of time that begins at the particular point and has a duration corresponding to the estimated duration of time that viewers are typically expected to discuss, through instant messaging the event of interest; determining an aggregate number of the identified instant messages that are received during the window of time; accessing a threshold; comparing the aggregate number to the threshold; conditioned on the aggregate number of the identified instant messages meeting the threshold, determining that the window of time corresponds to occurrence of an event of interest in the video, and calculating, based on the particular point in the timeline of the video and based on the determined amount of time associated with the anticipated event of interest, a start time and an end time of a scene of the video in which the event of interest occurs; generating metadata that specifies the start time and the end time of the scene of the video in which the event of interest occurs; and assigning the metadata to the video.