  In this chapter, we explore how gravitational interactions drive turbulent
flows inside planetary cores and provide an interesting alternative to
convection to explain dynamo action and magnetic fields around terrestrial
bodies. In the first section, we introduce tidal interactions and their effects
on the shape and rotation of astrophysical bodies. A method is given to derive
the primary response of liquid interiors to these tidally-driven perturbations.
In the second section, we detail the stability of this primary response and
demonstrate that it is able to drive resonance of inertial waves. As the
instability mechanism is introduced, we draw an analogy with the parametric
amplification of a pendulum whose length is periodically varied. Lastly, we
present recent results regarding this instability, in particular its non-linear
saturation and its ability to drive dynamo action. We present how it has proved
helpful to explain the magnetic field of the early Moon.
