To be an Uncle is the greatest thing ever. In my case, I have a niece from my little sister.
Being an Uncle gives complete freedom. You can make up your own science, tell stories from imagined history and lay the seeds for a healthy teenage rebellion.
It's common to hear the description "crazy" before the title. That alone allows you to bend rules. Grandparents can spoil a grandchild but are obligated to teach discipline as wise elders. The Aunt usually becomes the parent's "go to" second in command and is forced to maintain discipline.
An Uncle remains free without much required. Your main job is to teach new things a kid can add to thier chaos creating arsenal. You are the brother of one of the parents and this continues the long held teasing/torture tradition of siblings.
The Uncle code...support the parent's rules, but teach the loop holes.
When you hear a parent say in surprise..."Your Uncle taught you that?" Your job that day is complete.