Imaginary friends can often be seen eating spaceship-shaped cookies and random donuts. Imaginary friends who have been made real become a controllable member of the family. If you choose to, you can also make your sim's imaginary friend real, using a potion created with the Chemistry Lab.
Your sim will have to keep up a good relationship with his or her imaginary friend and interact with it a often so that it grows up. As a baby, of your sim can't do much with their doll, but they can do more and more as the sim - and, at the same time, the doll - grows up. Some toddlers created in CAS will also have a doll in their inventory. Occasionally, when a baby is born in your sim family, they receive a doll in the mail. Imaginary friends are a supernatural sim that spawns from a childhood doll.