Actually now that I read both definitions, neither is quite right. Perhaps a better metaphor would be dissolvable stitches – you need them when your wound is op

Actually now that I read both definitions, neither is quite right. Perhaps a better metaphor would be dissolvable stitches – you need them when your wound is open, but once it heals the stitches vanish. If anyone can help me out with a less gross version of the same metaphor I'd be grateful!