  A model for gas outflows is proposed which simultaneously explains the
correlations between the (i) equivalent width of low ionization and Ly-alpha
lines, (ii) outflow velocity, and (iii) star formation rate observed in Lyman
Break Galaxies (LBGs). Our interpretation implies that LBGs host short-lived
(30 +/- 5 Myr) starburst episodes observed at different evolutionary phases.
Initially, the starburst powers a hot wind bound by a denser cold shell, which
after about 5 Myr becomes dynamically unstable and fragments; afterwards the
fragment evolution is approximately ballistic while the hot bubble continues to
expand. As the fragments are gravitationally decelerated, their screening
ability of the starlight decreases as the UV starburst luminosity progressively
dims. LBG observations sample all these evolutionary phases. Finally, the
fragments fall back onto the galaxy after approximately 60 Myr. This phase
cannot be easily probed as it occurs when the starburst UV luminosity has
already largely faded; however, galaxies dimmer in the UV than LBGs should show
infalling gas.
