When I was travelling in China recently, I was attracted by the signboard by the side of land with grass. It tells the public in a subtle not to step on the grass. Look at the signboard. I shall explain what the Chinese words mean and the hidden message it tries to convey to you.
This is actually the play of psychology. If you ask a child not to do something, he will just do it to find why he is not allowed to do it. Hence, if you tell a child not to step on the grass, he will just do the opposite. Some adults behave like children too.