Bwalon, Artifact Energizer

{t}: Add or remove a charge counter from target artifact you control. Then you may
pay {x}{u/r}, where X is equal to the number
of charge counters on that permanent. If
you do, untap Bwalon, Artifact Energizer.
"It may take a few clangs and thunks to
get these hunks of junk whirring, but they
purr like kittens."