The site tour uses an example question to illustrate how sites work. By default, it is a question about preventing unicorns from eating daisies. I know that it's possible to change that question (some sites have done so) and that there are some pretty strict requirements on candidate questions in order to be used for that. (On one site where I'm a moderator, following the "edit question" link just leads to a message saying "no eligible questions".) I believe there is a length limit and that certain formatting is not allowed.
I've searched MSE and checked the moderator-only section of the help center but I can't find the specific requirements. What are they? If my site wants to change the tour question, what conditions must our proposed replacement satisfy?