The second patentability test requires that your invention must be "non-obvious". This test requires a high degree of objectivity which is not always easy for an inventor. The official explanation for non-obviousness is that "a person having ordinary skill in the art"* would not be able to easily figure out the same solution your invention presents.
*a fictional person who is considered to have the normal skills and knowledge in a particular technical field, without being a genius (Wikipedia)
The following explanation might help you understand this test. Check out this example of an invention that failed the "obviousness" test:
For example, in an early Supreme Court case, a patent claimed a doorknob made from clay with the
same shape as previously made knobs made from metal or wood. The clay knobs were fitted to a metal shank, where previously only metal or wood knobs were used. The substitution of clay for metal or wood may have made a better or cheaper doorknob, but the change only required the skills of a mechanic, not an inventor. Consequently, the invention was found to be obvious and not patentable.
(Scott R. Conley, Irrational Behavior, Hindsight, and Patentability: Balancing the “Obvious To Try” Test With Unexpected Results, IDEA Vol. 51 No. 2)
Check out this follow-up post:
Inventioneers Expand "Obviousness" Tip
