An invention has to meet a number of requirements to qualify for a patent. Before you file, it is worth checking that yours meets all of them.
1. Novel
Your invention has to be novel. Patents are only granted for inventions that had not been disclosed to the public anywhere in the world before the filing date. If your idea has already been described, discussed or sold publicly, there is no point in applying.
One application for a ship salvage method was refused because Donald Duck had used the same approach in a 1949 comic.
Modifications and improvements to an existing idea do qualify, though, so don't give up too soon. Even a small, clever change can be the difference between an average product and a very popular one, which makes a patent on that change extremely valuable.
2. Inventive
A difference between your idea and what is already known is not enough on its own. The difference also has to be non-obvious to an uncreative person who knows the technology.
Inventors often think their idea is simple in hindsight and assume anyone could have come up with it. But inventors are creative by definition. Patent law asks what an uncreative person would have done, so there is usually more scope than you would expect.
Using a welded joint instead of the usual bolted one may look obvious. But if welding brings unexpected benefits, it can certainly be inventive. If you have any doubts, talk to a patent attorney. Inventors are often far too hard on themselves here.
3. Industrially applicable
Because a patent protects technical ideas, an idea has to be capable of industrial application. For physical products this is rarely a problem, since almost anything physical can be made industrially.
Methods and processes can also be patented. They count as industrially applicable if they can be used in a technical or commercial setting rather than only at home.
In most fields this requirement is barely an obstacle. In life sciences it can be a real one. When patenting a gene, for example, you have to explain what its intended industrial application is.
