Sudden Intelligence

You have a character. This character is flat out dumb. He is either a Cloudcuckoolander, The Ditz, or even stupid to the level of Ralph Wiggum. Every bit of humor you get from this character comes from the unpossibility of him failing English or being clueless about how to open a pickle jar among other things.

However, after a few years you hit a snag. You're starting to run low on jokes regarding his sheer stupidity. You've overused jokes about them forgetting to breathe and now you need something different. After all, variety is the spice of life. How do you change that?

The usual answer is to momentarily give this character the Smart Ball. Completely out of the blue, your ditz spouts out a tidbit of knowledge regarding astrophysics or thermodynamics. Some writers, however, take this act Up to Eleven, by writing an entire episode regarding a character suddenly showing unnatural intelligence.

Obviously after five seasons of being The Ditz, you can't just have him become a master of string theory without someone wondering why he never showed it prior. Something has to cause this rapid change in intelligence level. A blow to the head, a scientific experiment, or alien technology shakes his brain up enough to unlock a level of intellect previous unheard of.

The result of this sudden intelligence often involves the character becoming an ace, solving everyone's problems without batting an eye. He becomes an episode-long, equal-opportunity Deus Ex Machina, nothing becomes challenging for him to figure out. But this ace-hood can get to his head, turning him into an Insufferable Genius. This can lead to a case of Achilles in His Tent, the other heroes' life is at risk, but he won't help them (or they won't ask for their help) due to their strained relationship.

Usually the episode ends with something causing the character to revert to his old ditzy ways. Maybe this danger to the other heroes hit him in the head and locked that intelligence away. Maybe the other heroes purposely tricked him into doing something to lose it. Maybe he realizes what a jerk he became and willingly gives it up in the name of mending broken bridges with his friend. Either way, he's back to being the lovable idiot we're familiar with by the time the credits roll.

Fan Works

 * Played with in the Harry Potter Fanfic Ron's "Stupid" by Jeconais, in which Ron Weasley shows up at breakfast one morning at Hogwarts with a calmer demeanor and a sudden case of Sesquipedalian Loquaciousness. The cause?  He claims that Harry has extracted his "stupid" and frozen it in the form of a beater bat...

Western Animation

 * In an episode of Jimmy Neutron Boy Genius, the titular character performs an experiment on Sheen to make him a genius. It's successful, but soon it becomes Gone Horribly Right, as Sheen's intelligence just keeps increasing. Eventually his brain becomes so powerful he masters telekinesis. Jimmy has to try to find a way to zap the intelligence back out of him before he takes over the world.
 * Inverted in another episode where he makes himself stupid
 * The Simpsons: After a crayon is dislodged from Homer's brain, he becomes a genius. But when he causes the entire town to turn against him after he causes the nuclear plant to be temporarily shut down. At the end of the episode, he gets Moe to stick a new crayon in his brain, reverting him back to being the old, stupid self.
 * SpongeBob SquarePants: During a day of horseplaying, Patrick falls down a cliff and suddenly becomes a genius. He alienates his friends, and eventually enlists Spongebob in trying to make him stupid again. Turns out
 * Regular Show: Tired of being made fun of for not having a high school diploma, Rigby takes a smart drink that makes him a super genius. Mordecai drinks it too and engages in a Escalating War to see who is smarter. Eventually they get so smart that everyone else is too dumb for them to comprehend them, and they have to find a way of dumbing down to normal again.
 * Chowder is given food to make him more intelligent after Mung Daal gets tired of him being such a scatter brain. It works a little too well, with Chowder even realizing he's a cartoon character in a dumb cartoon, then uses his brain to change the show into a more educational, albeit boring, one.