useJoint2d ties two bodies together, or a body to a point of the world: 'hinge' for the pendulum and the chain, 'rope' for the lamp, 'spring' for the weight, 'slider' for the lift that keeps upright with a spring under it, and 'weld' for the windmill's blades, turned by a hinge's motor.
Every point is a point of the world, where the nail goes. Click to drop crates on them.