  The angular momentum of galaxies controls the kinematics of their stars,
which in turn drives observable quantities such as the apparent radius, the
bulge fraction, and the alignment with other nearby structures. To show how
angular momentum of galaxies is determined, we build high (${35}\,\mathrm{pc}$)
resolution numerical experiments in which we increase or decrease the angular
momentum of the Lagrangian patches in the early universe. We simulate three
galaxies over their histories from $z=200$ to $z=2$, each with five different
choices for the angular momentum (fifteen simulations in total). Our results
show that altering early-universe angular momentum changes the timing and
orbital parameters of mergers, which in turn changes the total stellar angular
momentum within a galaxy's virial radius in a predictable manner. Of our three
galaxies, one has no large satellite at $z=2$; in this case, the specific
angular momentum is concentrated in the central galaxy. We modify its stellar
angular momentum over $0.7\,\mathrm{dex}$ (from $61$ to
$320\,\mathrm{kpc.km.s^{-1}}$) and show that this causes its effective radius
to grow by $40\,\%$, its $v/\sigma$ parameter to grow by a factor $\times 2.6$
and its bulge fraction to decrease from $0.72$ to $0.57$. The ability to
control angular momentum will allow future studies to probe the causal origin
of scaling relations between galaxy mass, angular momentum and morphology, and
to better understand the origin of galactic intrinsic alignments.
