Here's a hypothetical (for now) situation: we make contact with aliens out there somewhere. We're not able to visit one another physically, at least not yet, but we can choose one object to send to the aliens to explain who and what we are. The absolute best object we can send for this purpose is an automobile.
I'm not exactly sure why or how the aliens came up with this method, but those seem to be the rules we're working with. Let's say we know these rules because some automated probe ship landed on earth, and it's a sort of automated cargo-return vehicle. It's about the size of a small house, and it has a large door opening into an empty cargo hold. Some very clever pictographs or something made it clear that the aliens would like one something loaded into the hold that they can take for study.
And, since I'm on a roll here making stuff up, let's say we know the craft will leave in 24 hours, and there's no on-board life support. So, we need something in there fast, and we can't stick some adventurous explorers in there. It needs to be one object that says as much about us as possible.
That one object has to be a car. No other single object is as packed with telling, revealing information about humanity and our technology, biology, and culture as a car. Let's break down exactly why:
• Biometrics. If we can't send actual people, something that's designed to exactly fit and work with a broad range of humanity is the next best thing. The interior of a car gives a good sense of the scale of humans, the shape of the seats gives a lot of information about our general shape and range of motion, the locations of the controls and pedals give valuable inforation as to what we're able to manipulate and control, and the resistances of those various controls reveals our levels of control and general strength.
The audio system would suggest our ability to hear, and the dynamic range of the speakers would suggest the range we're able to hear. The design of the instruments (and the existence of windows) would suggest the ability to see, and the lighting chosen for the instruments would hint at what part of the EM spectrum we can see in.
The climate-control and HVAC systems will give a good sense of the range of temperatures human beings are comfortable existing in, the care taken to keep exhaust gases outside of the passenger compartment reveals what's poisonous to us as well.
Also, the fact that an internal-combustion engine requires oxygen to work, and that oxygen is taken in via intakes open to the outside atmosphere should reveal that the occupants of the car live in an oxygen-rich atmosphere as well.
The safety systems and airbags, seat belts and all that would show the limits of what humans can endure, and the high importance we place on preserving our lives.
And I suppose we can sneak a few photographs into the glove box, too.
• Technological development: I'm assuming we'd send a very modern, high-end, and advanced car, since we may as well show off how far our technical development has come. A car is possibly the most mechanically and electronically complex of any mass-produced item commonly sold, and as such it's a great encapsulation of humanity's technological development.
Think about all the electronics in a modern car, for example: there's many computers, there's cameras, LCD screens, LED and advanced projector-type lighting systems, proximity sensors, speed sensors, radio and satellite receivers, 12V outlets and USB ports and so much more. Think how much an alien race would be able to extrapolate from this: we have at least earth-orbiting satellite capability, our electronics manufacturing capacities, our grasp of mathematics, boolean logic, and all that good computer-related stuff.
Mechanically, there'd be plenty to be extrapolated as well. Our metallurgical development, the fact that we seem to have some reserves of complex hydrocarbons, and, along with that, the careful engine controls to make sure that the usage of those hydrocarbons is kept under reasonable control should suggest that that supply of hydrocarbons (our people call it "gas," or maybe "maize") has limits.
The emissions control system would also tell them that we both care about what we're putting into our atmosphere and we're trying to do something about it. Which may suggest something about humanity culturally, as well.
And, of course, the fundamental way everything is put together would offer a wealth of information as well. Fasteners, welds, castings, all tell stories of how we build things, about standardized parts and precision tools and the sorts of tools used to put it all together.
• Materials: There's so many different materials in a modern car, and all those things have tales to tell. Leather, if it's understood as animal skin, suggests the existence of large animals other than humans, and our control over those animals. There's woven fabrics that reveal what we find comfortable, there's plastics, and even wood, which would hint at what covers a good portion of the surface of our planet.
The way we choose to texture certain materials to resemble others (fake leather, fake stitching) should hint at the idea that some things are more valuable than others to humans, and the lengths that will be taken to fake those things. Which may, in turn, suggest concepts of status in our culture as well.
• Design: Hopefully, it will be clear that the way the car looks is not purely utilitarian. A bit of study should reveal that the body of the car is not the cheapest or most purely aerodynamic or efficient way to make it, which should reveal that other factors are involved in the design of the car. Those other factors are key to understanding parts of humanity's rich, complicated culture.
For example, if we send a two-seater sports car that could easily have room for more seating with a different body design, it should be clear that the decision to only seat two was deliberate, and the look of the car was selected for other reasons. The use of chrome and other decorative bits may hint at what we find visually appealing, and other fundamental design concepts, like bilateral symmetry, will probably keep alien grad students writing papers for many revolutions of alien worlds around alien suns.
• Performance and experience: Assuming the aliens manage to cram their gooey carapaces into the seats and wrap their feelers around the steering wheel, they should be able to see what Earth-cars can do. Depending on what we send, they should soon find out that humans like speed. I'm not sure exactly what they'll make of that, but that could lead to research that culminates in the human concept of "fun," which is one of humanity's best qualities.
The performance characteristics should also reveal things like what sorts of G forces humans can comfortably endure, and could also suggest the scale of the terrain these vehicles were designed to operate within.
There's even more besides these: they can learn our languages from the labels in the car, and many cars now even have synthesized speech as well. The turn indicators and brake lights and other warning lights suggest non-verbal communication methods, and the fact that some cars have devices specifically to enhance the engine sound in the passenger cabin should be good for lots of exciting alien theories on Human Behavior.
If anyone can think of a better object to explain humans to the aliens, I'm all ears, but I'm pretty sure you can't beat a car. In fact, we should probably get one handy and set something good aside now.
And keep it in a garage. Unless we want the aliens to have valuable data on pigeon digestive tract biology.