system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How is it possible that my car keys unlock only my car and not all the others? Is it theoretically possible that my key could unlock a second car somewhere on the world (given I'm close by)?

Edit: Apologies for the poorly phrased question. While the mystery of mechanical keys is fairly interesting I always figured that there would be a limited amount of key/locks available. My question was particularly referring to the (new) wireless keys!!
gpt
Most modern cars have 3 layers of security when it comes to keys.   Remote Identification, Physical Identification, and Encryption Identification.   

Remote identification is your keyless entry.  It’s like when you yell for your mommy outside a locked room, if she recognizes your voice, she will respond and unlock the door to let you in.  Voices can be similar, so if in the rare case some other kid sounds like you, she could be confused and let the wrong kid in.

Physical Identification is the cut key itself, or in the case of Push Button start, the physical proximity of the key.   When your mom opens the door, she would make sure you look like her child.  You have a unique face but doppelgängers or your twin brother could fool her into thinking another kid is you.   

Encrypted Identification is the RFID chip inside the key that the immobilizer has to sense to allow the car to run.   Even if you can duplicate the remote entry or physical key, the RFID reader will still need to read the encrypted code programmed to each key matches to the Immobilizer before the car is allowed to remain running for more than a few seconds.   This is extremely difficult to duplicate and very short range, so the real key would need to be within a few inches of the ignition to work.  It’s like your mom verifying your fingerprint or DNA to make sure you are you, even if you look and sound like her child.