system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: (USA) Why do people find it necessary to post "private property"/"no trespassing" signs on the fence around their property? Why doesn't the fence serve that purpose already?
gpt
In some cases it may not be obvious that you are on private property at all.  Many national forests and other large tracts of public land have small sections of private land that the government never bought up, or are adjacent to private land such that someone wandering around on the public land could conceivably wander onto private property without realizing it. And plenty of fences have been around for a long time and may no longer delineate property lines.  Plus lots of farms divide fields and pastures up with fences, though they don't mark the edge of their property.

And generally speaking you aren't trespassing until you are asked or told to leave by someone.  A No Trespassing sign covers that in most cases (though there are still times when people can come onto your property until you ask them to leave, even with your No Trespassing sign.  If your mail box is up near your house, your mail man can come up to deliver your mail, for example.