A while back I was discussing a school challenge of making a marble run out of junk that would last for 20 seconds release to finish. One would think this would be an easy task, but really it isn't- honestly! So Mr Inventor set to work on a new design, which actually ended up lasting even longer than 20 secs!!
Any way the task is not simple and in this presentation certainly not suitable for preschoolers, however for older children it is certainly a great problem solving challenge. For younger ones, perhaps shorten the time, and use larger junk e.g. cardboard tubes stuck together and leant against blocks or a climbing frame, extend and adapt where needed.
Remember the important part is the children's own thinking it is so easy to step in and help but doing so prematurely is not productive, instead prompt and scaffold with questions or simple statements to encourage a line of thinking.