  In simplified models of glasses we clarify the existence of two different
kinds of activated dynamics, which coexist, with one of the two dominating over
the other. One is the energy barrier hopping that is typically used to picture
activation, and the other one, which we call entropic activation, is driven by
the scarcity of convenient directions. When entropic activation dominates, the
height of the energy barriers is no longer the decisive to describe the
system's slowdown. In our analysis, dominance of one mechanism over the other
depends on the shape of the density of states and temperature. We also find
that at low temperatures a phase transition between the two kinds of activation
can occur. Our framework can be used to harmonize the facilitation and
thermodynamic pictures of the slowdown of glasses.
