Context-aware applications are applications that implicitly take their context of use into account by adapting to changes in a user's activities and environments. No one has more intimate knowledge about these activities and environments than end-users themselves. Currently there is no support for end-users to build context-aware applications for these dynamic settings. To address this issue, we present a CAPpella, a programming by demonstration Context-Aware Prototyping environment intended for end-users. Users "program" their desired context-aware behavior (situation and associated action) in situ, without writing any code, by demonstrating it to a CAPpella and by annotating the relevant portions of the demonstration. Using a meeting and medicine-taking scenario, we illustrate how a user can demonstrate different behaviors to a CAPpella. We describe a CAPpella's underlying system to explain how it supports users in building behaviors and present a study of 14 end-users to illustrate its feasibility and usability.