This paper is a case study of teaching the usability of interfaces. The students of the computer science learn how to design a prototype of an interface for ticket vending machines. Paper prototyping is used as a tool to prepare a mockup prototype. The prototype is tested according to several scenarios of usage. Finally, working interactive digital prototypes are built using dedicated software and hardware tools. Students work in teams in all stages of these projects. As a result, the computer science students develop practical skills in general designing of interfaces, team work, and creative use of analog techniques for fast development of interactive front panels.