How to tell when two objects are near each other?

I'm a newbie at Arduino and I'm trying to figure out if my project idea will work. I would like to have a system where I have several objects and they can tell when one is in proximity of another. So if I have Object A, B, and C, and I put A and C next to each other, it will send this information back to the board that A and C are close. I don't need to know how close, just a true or a false.

I have been looking through some of the available products and it seems like a couple things I could use are RFIDs or color sensors (if I make each one a distinct color). I can't really see how either of those things work though, so I'm not sure if they are plausible solutions.

I'm sure someone else must have done a project where they want to achieve this, so is there some known solution for this? A tutorial would be wonderful. Thanks!

It depends on what your sensing distance is and how aligned the units need to be.
The simplest is by using IR and flashing messages between the two systems.
However, I would probably do with an AC magnetic field and sensor coil a bit like the way an RFID reader works.
problem is you have to be careful with any system that your transmissions don't deafen the receiver.
I haven seen any writeups on this sort of thing though.

Thank you very much for your reply. I guess this project is over my head. I'm going to just try and do the same thing by plugging the objects onto a breadboard instead. Then maybe I can upgrade it one day in the future once I've learned more.