newtonian mechanics - Can you get a playground to swing from stationary

I feel this might be a FAQ but I would love a definitive answer.

Imagine a frictionless stationary idealised child's playground swing. If you are sitting on the seat of the swing, is it possible in principle to set it into motion by simply moving your body and pulling on the chains of the swing? If so, is there a simple explanation for how you can change your center of mass without an external force acting on you?

Answer

Yes. A swing does not depend on friction. The way you start up is by leaning back and pulling hard with your hands placed on the ropes. Your hand-hold positions effectively create a compound pendulum, so your mass is offset from null. By timing your shift in weight back to a "sitting-up" position, you essentially add energy to the pendulum system in phase and frequency with the full-length pendulum's oscillation.

I'm sure there are some pictures of this somewhere :-)

EDIT: I should have said, as Peter Shor did, that this makes use of gravitational force.