The relevance of data created in or about the IoT has a strong reliance on the context, especially spatiotemporal context, of the device and application perceiving it. To ensure that applications perceive data items that are relevant to the current context, it is necessary to restrict when each item is available. To control an application's perceptions of data availability, data items are often put in a group with other similar items, and a static rule is applied to determine when that data can be seen by applications. Such rules are fairly rigid, and the burden is on application developers to manage individual data items and their access policies, including making sure data distribution stays up to date relative to the context that influences data availability. We posit that the development of applications that need to access contextually relevant data can be greatly simplified by enabling a data item itself to control how and when it is available to applications. To realize this simplified programming paradigm, we introduce the datalet, an abstraction of a piece of data that understands its contextual relevance and dictates how (i.e., when and where) it is available based on that application's context. The datalet allows the application developer to focus on the application logic that relies on available data, without worrying about how to store, update, and distribute contextually-sensitive data to (distributed) application instances. To show how datalets are used by application developers to construct an application, we create an augmented-reality game that uses datalets to make elements of game play available based on the player's spatiotemporal context. The video of this demonstration is on YouTube at: https://youtu.be/snFhokswWpc.