With the emergence of the Internet of Things there are many applications in which the interaction is distributed over multiple devices. Developing applications in these scenarios is challenging because there is not enough knowledge and even less consensus on how to distribute interaction. But following the ongoing trends such as responsive web design, why not enable applications to seamlessly adapt interaction to take advantage of available devices at any moment? In this work we introduce the foundations of a new approach called Responsive Cross-Device Applications (RCDA). RCDA applies the idea of responsive Web applications distributing user interactions over the new cross-device ecosystem, taking into account the interactive capacities of devices and users.