If, say, your shower worked fine until today and then suddenly won't work. . .
If, for example, you try to push that little metal knob down on the faucet and it won't shut off the shower . . .
If, for instance, you pull and push and beat and wiggle the thing and it still won't go down . . .
you might just check for a small Christmas ornament that may have been been pushed up inside by someone whose fingers are much shorter than yours, but are still able to reach much farther back than you.
Not that I would know anything about that.