Like a child's toy, the book marks time in a literal manner, with steel balls that roll through a series of ramps and switchbacks. At the top left is a vertical slot from which balls are fed into the course. The first thing they hit is a blue toggle switch at the top of the page. As shown, its switched to the left, trapping the last ball dropped. The next ball through will knock it to the right and both balls will move forward on different paths. I borrowed this toggle trick from Matthias Wandel, a woodworker who has videos and explanations of his marble machines online.