The other day I transferred two batches of beer from fermenter to kegs. I always fill my clean kegs with starsan after cleaning the kegs and cleaning the kegerator lines. I then push the starsan through the lines into a container or in this case an empty keg that I had clean that was on deck for the next finished brew. This way the kegs are sanitized and purged of O2. The spare keg would just need the starsan pushed out with CO2.
This resulted, after hooking up the first keg of beer to a gas line, in two identical kegs on the floor next to the kegerator, one full of beer and one full of star San solution.
Can you guess based on the context of the title of this thread and the contents of this post which keg is now perfectly carbonated?