A mutation can create an alternative form of a phenomenon - a phenotype or trait - and we can learn a lot by seeing this alternative state. Once I saw a mouse with no eyelids. It simply had a membrane over the eyes. I found it fascinating that there is a single gene required for eyelids to develop.
Medical Science for the Curious : Why Study Medical Science? (A Guide to Choosing a University for High School & College Students, Their Career Advisors, and Parents)