  In the context of the energy landscape description of supercooled liquids, we
propose an explanation for the different behaviour of fragile and strong
liquids. Above the Goldstein crossover temperature Tx, diffusion is interpreted
as a motion in the phase space among unstable stationary points of the
potential energy, that is among saddles. In this way two mechanisms of
diffusion arise: mechanism A takes place when the system crosses potential
energy barriers along stable uphill directions, while mechanism B consists in
finding unstable downhill directions out of a saddle. Depending on the mutual
value of the efficiency temperatures of A and B, we obtain two very different
behaviours of the viscosity, reproducing the usual classification of liquids in
fragile and strong. Moreover, this scenario very naturally predicts the
possibility of a fragile-to-strong crossover when lowering the temperature.
