First to answer the question that started the thread:
I a mechanical because I always was fascinated by machines. I still am. I got my degree from the Univertsity of California, Irvine. I also was fortunate to work one summer as an intern for Rockwell Int'l on the B-1 program. After I graduated I started working here at Northrop Grumman. At the time I figured I would stay four or five years and then get out of aerospace. Somehow I'm still here, twenty two years later. I work doing the process tweaks that Andy Baker hates. But often these involve designing new tooling so I get to design stuff too.
Now to address the other topic that has come up:
In all fifty states to advertise your services as an engineer to the general public, you must have a Professional Engineer's license in the discipline you are practicing. As a Mechanical PE I can design an air conditioning system for a building, but I cannot design the building itself. That's a civil job and requires different expertise.
It is possible to become a PE without going to college. To do so you must first pass the Fundimentals of Engineering exam. I took it in my junior year in college and passed. Back then it was called the EIT (Engineer in Training) exam, but they cover pretty much the same stuff. All disciplines take the same FE exam. After passing the FE, you need to work under the supervision of a licensed engineer for four or six years. Four with an engineering degree and six without. Then you need to get references from several other engineers familiar with your work. At least one or two of these should be PEs. After all this you get to take an eight hour exam. If you pass, then you can legally call yourself an engineer. The exams are very broad. It is unlikely that a person who is just working someplace and trying to pass based on their experience would be able to do so. The questions generally cover all the corners of a good engineering curriculum.
So what about all these guys running around working at companies and calling themselves "engineers"? Most never bother to take the PE so how can they do that? First of all, their services are not marketed directly to the public. Their company develops products that are in turn sold to others. The company operstes under a "corporate license". Somebody there will be a PE and have "responsible charge" over the design of the products. If there is a liability issue with the product, that person will be on the stand in court and they'd better have some good answers.
As long as the "engineer" title is only used internally, it is acceptable for non-PEs to be called engineers by the company they work for. Sometimes the "supervision" can be pretty tenuous. There are three or four levels between me and the "chief engineer" on the programs I work. But then I can sign off on stuff myself if I want
Call yourself anything you want, but the Engineer title has a price tag. You can earn it through study and hard work, or you can just hang out a shingle. But the later will probably earn you at least a fine and possibly jail time.
ChrisH