Whatever you create in Juice, be it a robot or a windmill, will be known as a model. Models consist of beams connected together with joints. There are currently two kinds of joints, hinges and sliders.

You can add beams and joints to the model via the menu or toolbar. Once the parts are in place, you can connect beams with joints by clicking on them.

Adding motion takes a bit more thought. There are different types of motion available. For example, "sine" motion cause hinges to oscillate back and forth smoothly; "step" motion is similar, but with a pause on each side (your knees do this when you're walking). For simplicity's sake, everything moves back and forth at the same rate. You can vary the timing and range of motion for each joint independently.

I've found that creating a model usually goes pretty quickly, but it's a real challenge to get the motions right so the model walks without falling over.