  We introduce simple models, inspired by previous models for froths and
covalent glasses, with trivial equilibrium properties but dynamical behaviour
characteristic of strong glass forming systems. These models are also a
generalization of backgammon or urn models to a non--constant number of
particles, where entropic barriers are replaced by energy barriers, allowing
for the existence of activated processes. We formulate a mean--field version of
the models, which keeps most of the features of the finite dimensional ones,
and solve analytically the out--of--equilibrium dynamics in the low temperature
regime where activation plays an essential role.
