Why is there a mathematical task in the contact form?

The contact form on www.welcome.stuttgart.de contains a simple mathematical task which has to be resolved. This task comes under the classification of a so-called "Captcha".

A "Captcha" has been introduced in order to prevent spam in the guest books, forums, contact forms, etc. "Captcha" stands for Completely Automated Public Turing Test which can dissimulate between computers and humans (in German this is called "Vollautomatischer öffentlicher Turing-Test, um Computer und Menschen zu unterscheiden"). One must firstly understand that spam is not introduced manually, rather by so-called robots which scour websites for forms and then fill them in with their spam before sending the forms on to the recipient. "Captcha", which is not readable for most of the current robots, intercepts the spam and prevents it from entering the database.

The "Rechen Captcha", which is implemented on www.welcome.stuttgart.de, is not so widely utilised to date. It exploits the stupidity of robots which are not "clever" enough to do calculations. A simple calculation is generated, which can be solved in the head, and which then must be entered in a text field before being subsequently assessed for correctness.