You might have heard of the ultrasonic teenage repellent called the Mosquito.  The device was originally intended to clear away youths loitering in front of stores and other public places by emitting a high-pitched tone that is audible only to people below the age of 25.

Finally the youths found a new, youth-friendly purpose for this tone by using it in class as a secret notification tone for text messages.  This way they could text each other without the teacher knowing.

This is a good example of how the use of a functionality depends on:

• A context (initially in public spaces / then in class)
• A goal (initially to create peaceful environments / then to not be detected)
• And users (persons under 25 years).

Here is another example of contextual knowledge.

To denounce abuse and provide help to abused children, the Anar foundation created a publicity panel that displays a telephone number visible only to children under 10 years of age.  An essential information when the child is with a bully says the publicity campaign.