One line, and you can never lift it off the glass.
The constraint is the whole toy. One knob moves the stylus sideways, the other up and down, and the stylus never leaves the glass — so every drawing is a single unbroken line, and the only way to get to a new place is to draw your way there. Diagonals need both hands at once. That is why a staircase is easy and a circle is a party trick.
You can turn the knobs by dragging them round, or use the arrow keys: ← and → for the left knob, ↑ and ↓ for the right. Hold two at once for a diagonal.
Shaking really does clear it, and it is the only way to clear it. The drawing is kept in this browser between visits, so a shake is a decision.