Sanity Ball

When characters on the same show take turns being the Only Sane Man or Straight Man, but only one of them is sane at a time, that character is holding the Sanity Ball. Differs from the Smart Ball in that the Smart Ball gives a character the ability to problem-solve; the Sanity Ball gives its holder a rational perspective (and the knowledge that everyone around them is acting irrational).

Whenever the character holding the Sanity Ball gets a Heroic BSOD, or falls for Love Makes You Crazy, or gets Put On a Bus, or otherwise loses the ball, someone else on the show picks up the Sanity Ball and runs with it, becoming the new Only Sane Man.

Every faction has its own Sanity Ball, so the heroes may share one while the villains are sharing another.

Anime and Manga

 * The Slayers: The Slayers' Sanity Ball is usually tossed back and forth between Gourry and Zelgadis. The secondary Sanity Ball is usually toted around by Xellos (maybe that's what's on the end of his staff?), since he seems to be the Only Sane Villain. Usually.
 * Al and Leona in Dominion Tank Police tend to trade off the Sanity Ball rather frequently.
 * Beelzebub does have a few designated Straight Man characters, with Furuichi counting as the Only Sane Man amongst a cast full of delinquents and demons. However even the Cloudcuckoolander Oga comes out with bits of wisdom/cynicism if Furuichi decides to g off on one.

Comic Books

 * In Calvin and Hobbes, Hobbes generally carries the Sanity Ball when Calvin's interacting with him. When Calvin interacts with adults, Calvin's usually the one with the ball. When he interacts with Susie, though, it's usually by chucking the Sanity Ball at her and running away.

Calvin may hold the ball when his father is getting into one of his hobbies, but mostly the adults are a foil for his complete lack of common sense and some other qualities that don't exactly qualify him as the most sane character.
 * In The Sandman - Brief Lives, Delirium (the Anthropomorphic Personification of madness) acts generally insane and wacky while her older brother Dream serves as her rational and pragmatic foil. After receiving some distressing news, however, he starts to lose it, and Delirium has to pick up the Sanity Ball, since "one of us has to keep this thing going". Later on when describing the event, she seems unable to remember anything beyond saying "It hurt..."

Film

 * Return of the Jedi: The Emperor turns psychotic at the end, and tosses the Sanity Ball to Darth Vader. Vader then gets the ball, has his first moment of sanity in a while, and switches sides.
 * Characters in the Austin Powers films spend a lot of time passing the Sanity Ball around. For example, Scott Evil is usually the only sane man among the villains, but Number Two sometimes has sane moments as well.

Literature

 * Happens a lot in Discworld:
 * The witches' coven has a Sanity Ball, usually (though not always) carried by Nanny Ogg.
 * The wizards juggle two or three Sanity Balls, with the Librarian, Rincewind, Ridcully, and Ponder Stibbons being the most frequent holders. The Bursar had one at one point, but it's now considered too sharp to allow him to hold.
 * The Watch's Sanity Ball is usually carried by Vimes, but he'll toss it to Carrot or someone else once in a while, as in the That! Is! Not! My! Cow!! incident.
 * In Death's family, Susan or Death usually have the ball, but Albert takes it occasionally, such as when Death goes on vacation in Mort.
 * In The Hitchhiker's Guide to the Galaxy, Arthur starts off with the Sanity Ball, but he soon gets out of his depth. Ford Prefect carries the ball for most of the story, though Trillian/Tricia gets to hold onto it sometimes too.
 * Considering that Ford is still pretty out there, this is saying something.
 * Stationery Voyagers: Each of the Voyagers gets a chance to hold the ball, but some hang on to it longer than others. As The Atoner, Liquidon often gets it for the shortest amount of time.
 * Common in PG Wodehouse's books. Bertie Wooster is a semi-frequent recipient, sometimes borrowing it from Jeeves or otherwise sharing it with him.

Live Action TV
"Shawn (to Gus): Someone needs to hold the sanity bag in this partnership. And, frankly, I miss the days when it used to be you!"
 * Lost: This happens a lot, especially between Jack and Locke, who seem to alternate between being the voice of reason and acting insane. Usually when Jack is acting crazy it's Locke who is the sane man, and the other way around when Locke is acting crazy. And for most of season five it was Sawyer who had the sanity ball, oddly enough, as Jack had been relegated to second string and Locke was.
 * In House, Wilson or Cuddy both hold the sanity ball for House usually. This enables House's brilliance/insanity to counter his teams' mediocre/pragmatic approaches with the sanity ball holder to act as referee. But the ball is sometimes passed, to 13 (when Wilson is dealing with Amber's accident) and to House (when Cuddy is dealing with sick babies). House is reliably crazy when it comes to medical diagnostics but with regards to each team members individual problems (Cameron marrying chase, Foreman switching 13 off placebo, Taub confessing his affair, etc.) House has been sought for his reasonable opinion.
 * Farscape: Early on, Crichton is the Only Sane Man. As the series progresses, John becomes a bit less sane, and Aeryn and even D'Argo are assigned occasional ball-carrying duties.
 * Common on Arrested Development - Michael holds it more often than anyone else, but promptly passes it to George Michael whenever he deals with the kids, and George Michael and Maebe play catch with it when they're together.
 * Lampshaded in the Psych episode "Not Even Close...Encounters" when Gus and Shawn try to refrain from indulging in alien theories during their investigation:

"Peter: So you're not buying that it's her husband's ghost?
 * In Doctor Who, the characters seem to be playing a constant game of hot potato with their Sanity Ball. Sometimes the Doctor, sometimes a companion, sometimes a companion's family member, currently, Rory seems to be holding it the most.
 * Fringe: Tossed around quite frequently, as everyone has a slightly different theory on where to draw the line between the very, very strange and the just plain impossible.

Walter: There's no such thing as ghosts.

Peter: That's where you draw the line? Ghosts?"


 * Canadian teen sitcom Mr. Young passes this between Adam and Echo.

Webcomics

 * The Sanity Ball is bounced around in Jayden and Crusader a lot. Jayden started out with the ball acting as the straight man to Crusader, but eventually her anger issues and Zoiberg fetish made her pass the sanity ball to Crusader, who passes it off whenever he has something crazy to do. Third is arguably so sane he's crazy.
 * In The Order of the Stick, a good dozen or so characters have passed the Sanity Ball around. Roy usually carries it, but when he was killed, Haley and Durkon picked it up for their respective parties. Elan occasionally takes the ball for a bardic Genre Savvy moment.
 * Used a lot in Eight Bit Theater. Usually, both Thief and Black Mage can be relied on to be sane, but Thief will constantly behave irrationally as part of his own schemes, or Black Mage will go crazy and try to kill everyone. When these both happen at once, Red Mage will step in as the voice of reason, but usually his plans turn out to be even worse. But they tend to work anyway. And when the three of them are caught up in bickering and semantics, Fighter becomes the reasonable one, because he's too stupid to realize what's going on.
 * Or, when they're all being crazy, White Mage.
 * The Ciem Webcomic Series tended to have designated ball holders. If Candi herself wasn't holding it, then it was Laurie. Or Miriam. Or Denny. Or Imaki. Or Wilbur. Pretty much literally everyone else, however, was either one-dimensional or completely insane on a regular basis. Arfaas gets to act somewhat normal whenever around Musaran, if only to emphasize what an off-the-chain rabid monster Musaran is.

Web Original

 * Some of the characters in Red vs. Blue tend to throw the Sanity Ball back and forth with each other, particularly Grif and Simmons, and also Church and Tucker.
 * Rooster Teeth Shorts: The Sanity Ball seems to be in the hands of someone different every episode. Most obvious with Joel, who tends to swing between being the Only Sane Man and being a complete Cloudcuckoolander.
 * Matt is probably the most consistent with it, as even when he's "crazy", he's usually right or out-crazied by someone else.
 * In Yu-Gi-Oh the Abridged Series, the sanity ball is usually held by Yugi, but is passed around liberally; in recent episodes, even Tristan has held it.
 * Homestar Runner moves the sanity ball so much its hard to tell who's smarter than who. Out of the whole cast, Bubs, Strong Bad, Strong Sad, and Marzipan would probably be among the smarter members of the cast, but even then it changes. Marzipan can go from being the only sane person and within seconds become a complete lunatic. Even Strong Sad, who is usually depicted as the Only Sane Man, has his moments of craziness.

Western Animation
"Amy: Bender, are you crazy?
 * Star Wars: The Clone Wars. Only a few clone troopers (from Hidden Enemy and Deserter) seem to realize that they are mostly used just to senselessly charge into battle and die.
 * On Jimmy Two Shoes, either Jimmy or Heloise hold the ball, depending on the joke. When interacting with Jimmy and Beezy, Lucius will occasionally hold.
 * SpongeBob SquarePants: Spongebob tends to act like an actual adult whenever he and Patrick are the only characters on screen, if only to emphasize exactly how brain dead of a person Patrick really is. However, expect him to drop the Sanity Ball like a rock once Squidward gets involved.
 * Invader Zim: Zim's robot GIR, dispite his processor being mostly made from trash and spare change, has occasional moments of clarity. The first would be when he question's Zim's plan to kill Dib in the past with time-travel, the second is when Zim decides to stop an explosion by undoing the timefield that's slowed it down.
 * Dib can be manic and gullible or the sarcastic Only Sane Man, depending on the situation. The former is more common when he's talking to Gaz (who alternates a bit herself) while the latter usually occurs with Zim (who's almost always crazy).
 * Tale Spin constantly trades the Sanity Ball around, most especially between Baloo and Rebecca. On average, both have an equal amount of idiotic or callous moments, usually with the other acting as The Straight Man to it all.
 * A common occurrence with Stan and Kyle in South Park. While they CAN at times both play the Straight Man / Only Sane Man in an episode, if one of them is in possession of the episode's Idiot Ball, the other almost undoubtedly will play the sane one.
 * Generally shared between Leela and Fry in Futurama, with a few rare exceptions.
 * Lampshaded in Into the Wild Green Yonder:

Bender: Nah, it's Fry who's crazy in this one."