Cosmo and Wanda cannot be seen by anyone otherwise they'll have to go away forever; since it's Halloween, they dress up in costumes so they can be seen without anyone knowing they're fairies.
Example of:
Loophole Abuse
Cosmo and Wanda cannot be seen by anyone otherwise they'll have to go away forever; since it's Halloween, they dress up in costumes so they can be seen without anyone knowing they're fairies.
