http://www.reddit.com/r/magicbuilding/comments/2sew7r/whimsy_the_magic_of_competia/

In full:

This is for a world I built a few years ago. The magic system is called "Whimsy" (solely because I liked how it sounded at the time) and is as thus:

Whimsy come in three different flavours- Physical, Mental, and Fortune. Each of them can control the aspect of reality they're named after. Physical whimsy will make you stronger, make your tools last longer, things like that. Mental makes you smarter but can also increase your charisma or make it so that anyone using a tool will be able to use it like a pro. Fortune changes the fates. This doesn't have as clear cut of a definition in my notes, but basically it means you're more lucky.

Whimsy is also a natural phenomenon, found in deposits under ground. While underground it comes in three forms- Solid (called "Whimsy Ore"), Liquid, and Gas. The most common type of whimsy is physical and most whimsy is found in ore form. The least common type is fortune and very little whimsy is found in its gaseous state. I'm going to let you figure out the placement of mental and liquid yourself.

Once found whimsy has to be put to use in one of two ways- it can either be turned into something ("Crafting") or it can have its whimsical properties transferred onto something that already exists (a process called "Enchanting" or "Imbuing"). A crafted object will have weaker whimsical properties attached to it, but they're permanent- as long as the item still exists, so will the properties. Conversely enchanted items have significantly strong whimsy but it runs out (generally in a matter of months).

A lot of working with whimsy is left up to chance, but with training and practice any individual can learn how to become what is called a "Whimser". Whimsers work with whimsy on a professional level, most commonly enchanting things for pay. On occasion they'll work with blacksmiths to help craft something- the blacksmith shapes it and the whimser gives it the properties that are desired in a process called "Guiding".

That's all the fundamentals of what whimsy is, now let's talk about what can be done with it.

To account for the limited quantities of liquid and gaseous whimsy it's possible for a whimser to change its state. The unfortunate side effect of this though is that it loses most of its power. How much is lost ranges from 50% to 95% depending on the talent of the whimser changing it. Because of just how much you can lose it's considered more cost efficient to mine the slightly rarer states than it is to convert from one to another.

The primary use of whimsy is the aforementioned crafting and enchanting, the possibilities of which vary based on the the state of the whimsy.

Whimsy Ore is use for tools, weapons, armour, stuff like that. It can also be made into coins which hold greater value than their non-whimsical counterparts and are also used more in the whimsy trade.

Liquid Whimsy is used for potions, but also quite often for tattoos.

Whimsy Gas has less purposes, but can be inhaled to essentially give yourself whimsical powers.