  We conduct two-dimensional hydrodynamical simulations of jets expanding in
the intra-cluster medium (ICM). We find that for a fat, i.e. more or less
spherical, bubble attached to the center to be formed the jet should have high
momentum flux and a large opening angle. Typically, the half opening angle
should be >50 degrees, and the large momentum flux requires a jet speed of
\~10,000 km/sec. The inflation process involves vortices and local
instabilities which mix some ICM with the hot bubble. These results predict
that most of the gas inside the bubble has a temperature of 3x10^8<T<3x10^9 K,
and that large quantities of the cooling gas in cooling flow clusters are
expelled back to the intra-cluster medium, and heated up. The magnetic fields
and relativistic electrons that produce the synchrotron radio emission might be
formed in the shock wave of the jet.
