  One of the most important experimental results for proton-proton scattering
at the LHC is the observation of a so-called "ridge" structure in the two
particle correlation function versus the pseudorapidity difference $\Delta\eta$
and the azimuthal angle difference $\Delta\phi$. One finds a strong correlation
around $\Delta\phi=0$, extended over many units in $\Delta\eta$. We show that a
hydrodynamical expansion based on flux tube initial conditions leads in a
natural way to the observed structure. To get this result, we have to perform
an event-by-event calculation, because the effect is due to statistical
fluctuations of the initial conditions, together with a subsequent collective
expansion. This is a strong point in favour of a fluid-like behavior even in
$pp$ scattering, where we have to deal with length scales of the order of 0.1
fm.
