I like meta-normative signs, signs that tell you to obey other signs (like this sign telling pedestrians to obey traffic signals). The photo above is of a sign that is kind of like that. See the coaster on the table? The presence of a coaster is an implicit plea for you to use said coaster, and its function is obvious: to protect the table. Except in this case, someone a) figured you wouldn’t know to use the coaster, and b) wouldn’t know its function. So they told you. Politely.
This normative sign also illustrates one of my (perversely) favourite uses of normative signs: to intrude into your brain when the objects they’re protecting should just be designed so as not to require instructions for proper care.
Thanks to Wayne Norman for the photo.