  It is shown that many properties of Ly-alpha forest absorbers can be derived
using simple physical arguments. Analytical expressions are derived for the
density and the size of an absorber as a function of its neutral hydrogen
column density, which agree well with both observations and hydrodynamical
simulations. An expression is presented to compute Omega_IGM from the observed
column density distribution, independent of the overall shape of the absorbers.
Application to the observed column density distribution shows that at high
redshift most of the baryons are in the forest and suggests a simple
interpretation for its shape and evolution.
