Patent law states that the invention must be
1. Both novel and useful
2. More than an “obvious” alteration of an existing invention. In your example, it would depend on the novelty of your pizza-cutter aerator. You're right in that the combination might be considered "obvious to try" (in patent-office lingo). Patentability in this case would lie in the details of your mechanism. So, say, in addition to the pizza-cutting mechanism, you invented a new blade shape or a new twist that makes your aerator aerate better than any existing aerator, you’d have a chance at patentability.
It all depends on how your invention compares to the prior art. Prior art is not just patented inventions, but any product, or anything that’s been publicly disclosed, even if it is not currently in commerce.
Great that you joined the boards. Your advice is greatly sought after, I only met one person so far with this kind of expertise - he is very sought after as you can imagine. I deal with patentability all the time and do it as early on in the concept stage as possible. I will aim for a specific question. One question I run into is how simple an invention can be to be patentable. For example, a random idea I had was to improve an aerator rake by outfitting it with pizza cutter blades instead of the static teeth. The wheels exist, it is novel, but is it too obvious and simple? Sometimes a new invention arises by
outfitting something existing with one extra simple element, that is why this question regularly pops up.
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