In the recent years, the issue of cooperation, integration, and coordination between information peers in a networked environment has been addressed in different contexts, including data integration, the Semantic Web, Peer-to-Peer and Grid computing, service-oriented computing and distributed agent systems. One of the main problems that arises in such systems is how to exploit the mappings between peers in order to answer queries posed to one peer. The goal of this paper is to present some basic, fundamental results on this problem. In particular, we focus on a simplified setting based on just two interoperating peers and we investigate how to solve the so-called ``What-To-Ask'' problem: find a way to answer queries posed to a peer by relying only on the query answering service available at the queried peer and at the other peer. We show that a solution to this problem exists in the case of peers based on a basic ontology language and provide an algorithm to compute it. We also show that, by slightly enriching the ontology language, the problem may become unsolvable.