coursework - Setting exercises and assignments when everything is on the web

These days there is so much material on the web that it is difficult to set good exercises/assignments for students, because they often find the questions on the web simply by googling (or else ask at a site such as http://cs.stackexchange.com).

How do people deal with this problem? How do you generate unique assignment questions?

For the record, the area I teach is computer science.