In university, you're not paying for your seat in the class, but rather for the opportunity to write the exams and get a grade (save for lab-oriented classes, w

In university, you're not paying for your seat in the class, but rather for the opportunity to write the exams and get a grade (save for lab-oriented classes, where equipment and materials are limited). It's quite easy to simply walk in and audit a class, and I remember quite a few people doing just that when the classes I was attending were being taught by fairly well known professors.