There are three major requirements for patentability: The invention must be Novel, Useful and Not Obvious. Lets take a closer look at each of these criteria:
Novelty
To start with; in order to be considered patentable, an invention must be “Novel”. This means that the invention was never described in a publication, used or for sale in public, or patented by others before you invented it. To determine if the claim meets the novelty criteria, you must do an exhaustive “prior art” search for every element of your invention claimed to be novel to ensure that the invention has not been created and/or publicized previously. If even a single element is not described in a prior art claim, the claimed invention can be considered novel. For example, a claim might say “A table having a square top and with legs protruding from each of four corners” If a prior art document describes a table with a rectangle top and legs protruding from each of four corners, the claimed invention is novel because rectangle is not the same as square. You could argue that it is almost the same, but that is a matter of obviousness, not novelty.
Usefulness
The next criteria to consider is whether the invention is “Useful”. This generally means that the invention does something, anything at all. Very few applications are rejected on the grounds of “usefulness”. Generally, an “invention found “not useful” would be something along the lines of “perpetual motion” inventions or chemical compounds and gene sequences with no known utility. “Useful” does not require commercial marketability and just because you have been granted a patent is not a guarantee that the market will welcome your invention. A a couple of examples of patents that passed the USPTO “usefulness” test, but haven’t done so well on the market:
Furniture in the way? Tired of moving it to clean or just walking around it to get where you are going? Consider US 4,888,836 which provides a way to simply float it out of the way until it is needed!
Or how about US 5299356, a utensil that regulates the number of bites consumed during a specified period of time for dietary purposes. Simply set your parameters based on calorie or fat content of the food and this handy device times your eating, green light for go and red for stop. I feel thinner already!
Obviousness
In the words of Galileo; every invention seems obvious to the inventor, after it’s been invented. To pass the obviousness test, an invention must not be an obvious development from what has gone on before in the judgment of a Person Having Ordinary Skill in the Art. 
To better understand the obviousness test, lets enlist the help of our expert, Mr PHOSITA, who knows everything there is to know in his field and can tell us if he would have known to combine previously-existing elements to result in an invention, without having seen the patent application first.
Q. Mr. Phosita, regarding US 4,022,227 METHOD OF CONCELING PARTIAL BALDNESS: As an Person of Ordinary Skill in your Art, could you have anticipated this three way comb over, using only the hair on a persons head, as a method of concealing baldness?
A. I remember as a young child, that my grandfather had exactly the same hairstyle, then my father some years later and I anticipate that this hairstyle runs in my family.
Now, how did that one get by?!!
Objective Evidence
Additionally, Objective evidence or secondary considerations such as unexpected results, commercial success, long-felt need, failure of others, copying by others, licensing, and skepticism of experts are relevant to the issue of obviousness and must be considered in every case in which they are present. When evidence of any of these secondary considerations is submitted, the examiner must evaluate the evidence. The weight to be accorded to the evidence depends on the individual factual circumstances of each case.
Summary
When all is said and done, you must first meet each of the prescribed criteria to win your patent. Further, to have any real value, your invention must have some practical appeal to the market. 
Consider US 5356361 Zero Gravity Workout Device: An exercise device especially suited for zero gravity workouts. I see the mass market appeal of this device for future moon colonies…
For more Frequently Asked Questions on Patenting and the Patent Process, please visit our Patent Tools Page.
One Comment
In 20 years the zero gravity workout device could be in high demand. with the rate of development we will be constantly surprised by new things in the future. There will never be a time when there is nothing left to invent.