Content-based contextual reactions for posts on a social networking system

A social networking system allows users to more easily and appropriately react to posts made by other users. A post-including its text, photos, and/or location-is analyzed to determine one or more contexts of the post. Supervised machine learning techniques may be employed to determine the contexts of posts. Based on the context(s) of the post, a contextual reaction icon is selected for inclusion in the options of reactions from which users can select. A post can have multiple contextual reactions. The contextual reactions for a post may change over time as comments are made to the post or the content of the post is otherwise altered.