to make stuff up

An activity of the mind anyone can indulge in.
It is the act of simply constructing an entertaining story or otherwise for the amusement of yourself and/or your peers.
It is essentially an elongated White Lie in story mode.

For increased effectiveness the teller is required to have excessive creativity and/or a good imagination

Making stuff up usually serves to occupy the brain of the subject when he/she has nothing better to do with their time and has grown tired of using their creative side and/or imaginative ability to construct a realm where having everlasting hot sex with Eva Longoria, kinky pornstar sex with Priya Rai or even a mixture of both with some other super gorgeous lady, is a possibility, since those things can only happen outside the realm of reality.
Try it.

When you eat dirt for the first time and leave your family to build a shack in the woods