Punched cards were one of the first methods of entering data into computational machines and early computers. Each card would have holes punched through in a certain sequence to represent a form of data, which was then fed into a machine and "read" as input. Although we don't deal with those today, the following project is an excellent reproduction of the theory in simple form, and would be great to use in an educational context to show people how "it used to be done" - as well as general project construction. For example:
To recreate this is quite simple, the Arduino is just reading two digital inputs which are triggered by the punch card and a simple reader. This could also be used to create simple electronic voting systems for fun in the classroom situation.
For more information, the Arduino sketch and further inspiration visit this site.