Sometimes I find existing inventions that are identical to the ones invented by my clients. When I do, I ask my clients to tell me whether they think there are any differences between their invention and the one already existing. This is a pretty tough question to ask because most clients are already in the mindset that their invention is novel and therefore patentable (see my other article, "Can't Buy Everything..."). Usually clients will point out the smallest details to try to convince me that there are differences between their invention and the prior art. (But really, I'm not the one they should try to convince. It's the patent examiners we should be trying to convince.)
Some clients tell me, "Well, mine has an additional pocket," or "Mine has two pockets. This one only has one." Others tell me, "Mine uses a rechargeable battery."
But do these small differences really distinguish one invention from another? Depends. When there is a "real difference" between two inventions, one invention is said to be "patentably distinct" from the other invention. Patentable distinction means that the distinguishing feature is not anticipated or obvious in light of the existing inventions (aka prior art). Anticipation is when every feature of your invention is found in the prior art, and obviousness is when your invention is an obvious combination of two or more prior art. So if the difference is nothing but a common subject matter, there is no real difference between your invention and the prior art.
Going back to the above example about an invention having an additional pocket, it's probably unlikely that adding a pocket or two or three to something like a shirt is going to make the invention patentably distinct. Why? Because shirts with pockets already exist, and having more or less pockets doesn't really make a big difference.
Similarly, using a rechargeable battery doesn't make an invention patently distinct from another invention that uses a regular battery, a solar powered charger, or another type of existing power source. This is usually because inventions relating to electronic devices claim something broad like "a power source," so that it encompasses various types of power source. Additionally, a power source is a power source is a power source. You have to power your invention somehow, so it's pretty obvious that you'd use other existing energy source to supply power.
This might be a little discouraging for some inventors because it means that they have to go back to the drawing board. But I think this is good for technology advances. Also, ensuring that each invention is meaningfully distinct from another invention helps maintain the integrity of the patent system.