We don't even know where you'd start with designing something like this, but 22-year-old builder Shadowman39 is some kind of K'nex genius. He started to build this incredible elaborate K'nex ball machine, called the Citadel, in January 2013, and finished in June 2016. The really impressive part? It only took about a year altogether -- he took a break in between.
According to his YouTube channel, it is has five lifts, three motors, and stands 8 feet (2.43 metres) high, with a width of five feet and three inches (1.6 metres) and depth of four feet and two inches (1.27 metres). In all, it contains around 40,000 pieces. He hasn't uploaded a list of parts or set of instructions yet (according to a post on Reddit, that will have to wait until the machine comes apart), but there is a work in progress photo series, as well as a series of the finished build, available on his Flickr page.