The conventional assumption made in the design of communication systems is that the energy used to transfer information between a sender and a recipient cannot be reused for future communication tasks. A notable exception to this norm is given by passive RFID systems, in which a reader can transfer both information and energy via the transmitted radio signal. Conceivably, any system that exchanges information via the transfer of given physical resources (radio waves, particles, qubits) can potentially reuse, at least part, of the received resources for communication later on. In this paper, a two-way communication system is considered that operates with a given initial number of physical resources, referred to as energy units. The energy units are not replenished from outside the system, and are assumed, for simplicity, to be constant over time. A node can either send an 'on' symbol (or '1'), which costs one unit of energy, or an 'off' signal (or '0'), which does not require any energy expenditure. Upon reception of a '1' signal, the recipient node 'harvests' the energy contained in the signal and stores it for future communication tasks. Inner and outer bounds on the achievable rates are derived, and shown via numerical results to coincide if the number of energy units is large enough.