A hybrid pushbike / compressed air powered bike.
Step 1: find and modify an air tank.
this is an old propane tank. this turns out to be a very bad idea. although the propane was emptied out, the really heavy mercapton (stinnky sulfur based molecules) were left in the bottom of the tank. even after 20 odd air refills, it still smells awful to run air from this tank. hence the nickname - "stinkbike"