Fagiggly Gland

The Fagiggily Gland is what makes fairies able to shape shift to different objects or creatures to disguise themselves on Earth. Fagiggly glands can fail which will cause them to need a transplant. If the fairy has no siblings, they need to get their transplant from their Anti-Fairy.

Description
The Fagiggily Gland is a star shaped organ that every fairy has. It is the reason why fairies are able to shape shift into different objects to diguise themselves. Fagiggly failure is seen as an embarrasing topic for fairies. As seen in The Gland Plan, if a fairy is suffering from Fagiggly failure, they can no longer be fairy god parents because it would reveal their identity. When Cosmo was unable to control his shape shifting, he often transformed to objects that hurt Wanda such as a fork, jackhammer and chainsaw. Fairy babies have to be taught how to shape shift, as seen in Sooper Poof, when Poof's incorrect shape shifting reveals him as a fairy but is luckly mistaken for a super baby.

Disguises
Cosmo and Wanda specifically have disguised themselves as many different animals and objects to be with Timmy. They have also used their diguises to hide timmy, help him in some task or an object so they can be around Timmy's parents or friends. They usually keep their trademark colors of Cosmo being green and Wanda being pink.

Faires as Pets
Many fairies disguise their identity as fairies by looking like a pet of their god child. Although Cosmo, Wanda and now baby Poof, appear as many different types of pets for Timmy, they remain unquestioned and undetected. Other fairies such as Juandissimo Magnifico, Remy Buxaplenty's fairy, takes the shape of a purple ferret as seen in Fairy Fairy Quite Contrary. When Jorgen Von Strangle was Timmy's temporary fairy, he was a rhinoceros.

has as pets, Cosmo, Wanda and now baby Poof commonly appear to be Timmy's gold fish. When Cosmo and Wanda were Denzel Crocker's god parents, they were seen as his parrots.

At School
Timmy usually takes Cosmo and Wanda to school with him, commonly being his pencils, pens or characters on his book covers. Notably, Romeo and Juliette. He has also been on astronauts on Timmy's magic lunch box where he wishes for what he wants for lunch. When Timmy has missed the bus, Wanda and Cosmo turned into a bike and bell for him to ride to school, which wasn't exactly what he had wished for when he wanted a ride to school. They also were an art project called "Idiots in Love" when Princess Mandie encases them in carbonite. 

Most Common Forms
Besides their fairy form, Cosmo and Wanda frequently take the forms of balloons, squirrels and pencils and pens.