Most of us learn it at a young age. Even dogs know it.
Now, the High Court has given it legal substance in the recent Banerji decision (Comcare v Banerji  HCA 23).
In this case, the High Court found that the dismissal of a government employee for airing negative views about her employer on Twitter was lawful. The Court made this finding even though the tweets were posted anonymously, using an alias.
Even though the Banerji decision ultimately turned on the statutory obligation owed by a public service employee, there is no reason why employers cannot prohibit an employee from posting negative comment about the employer online and then dismiss that employee if they breach this condition.
This decision should be setting off alarm bells for employees everywhere.
What am I posting...? Will it get me in trouble? What impact might the post have on my employer?
In a world controlled by fiscal economics, where one harsh review can send public organisations into a tailspin, those organisations should be able to stop their own employees from harming their reputations.
After all, the internet is forever and some stains never come out in the wash.