The thick disk of our galaxy displays kinematic and chemical properties that are intermediate between those of the halo and the (thin) disk stellar populations. Not all disk galaxies have a thick disk. A theory of the origins of a thick disk can potentially provide insights into the physical state of our galaxy in its infancy. Levitation, a process that relies on adiabatic capture into resonance of stellar orbits in a growing disk, is presented as a plausible formation mechanism; a 2:2 resonance between vertical and epicyclic oscillations drifts to large vertical energies as the disk grows adiabatically. Resonant stars levitate several kiloparsecs above the plane, forming a thick disk whose spatial distributions, kinematics, and ages leave unique observational signatures on the sky. The same process can also produce the disk globular cluster system.