p.
  Interacting with Newton is easy -
  just inject Forces, Particles, or Constraints into the system
  based on your user's behavior.

pre.CodeBlock.
  var mousePin = null;

  renderer.on('pointerdown', function(point) {
      var particle = sim.findNearest(point, 30);
      if (particle) mousePin = sim.add(Newton.PinConstraint(particle, point));
  });

p.
  In this case, we've added a PinConstraint to represent the user.
  When the user clicks on a Particle,
  we bind the Particle to a new constraint, which we update to follow
  wherever the user moves her mouse.