The art of software testing

In the introduction to the book "The art of software testing" it poses a simple program and ask how many test cases you can think of...

"The program reads three integer values from an input dialog. The three values represent the lengths of the sides of a triangle. The program displays a message that states whether the triangle is scalene, isosceles, or equilateral."

If you write down all the test cases you can think of, you will most likely be surprised how many things you don't think of. (they provide a list of test cases in the link)