  A simple Ising spin model which can describe the mechanism of advertising in
a duopoly market is proposed. In contrast to other agent-based models, the
influence does not flow inward from the surrounding neighbors to the center
site, but spreads outward from the center to the neighbors. The model thus
describes the spread of opinions among customers. It is shown via standard
Monte Carlo simulations that very simple rules and inclusion of an external
field -- an advertising campaign -- lead to phase transitions.
