http://qoal.*******/js1k_pipedream.html
^made by QOAL in a contest to make something interesting without going over 1k. Zoom in; it gets easier to play if you do (ctrl and scroll wheel on firefox).
You have to make a "road" for the blue line. You can see what pieces are coming on the right. You can replace pieces by clicking on them again. You'll get the hang of it.