Pure functions file with header file always resulting in undefined reference

I'm trying to create a general functions file for myself. I want to do it right so no #include .cpp but .h files. However I result in undefined references all the time. I reproduced it with the following three files:

main.cpp :

#include <iostream>
#include "functions.h"
using namespace std;

int main()
{   
    cout << addNumbers(5,6);
}   

functions.h :

#ifndef FUNCTIONS_H
#define FUNCTIONS_H

int addNumbers(int x,int y);

#endif

functions.cpp :

#include "functions.h"

using namespace std;

int addNumbers(int x, int y)
{
    return x+y;
}

The files are all in one folder. I'm using Linux Mint, geany and c++11. Compiling results in the following error:

main.cpp:(.text+0xf): undefined reference to `addNumbers(int, int)'

Unfortunately I only found similar problems concerning classes online. I have come so far as to understand it is a linking problem though I have no clue about that part of the compiling process. Adding the function to the .cpp before main() or after with prototyping works fine. This question Undefined reference C++ seems to be similar but I don't understand the answers.

My questions are:

1. 
   

   How can I solve this? (I do not wish to wrap the functions in a class or add them to the main.cpp)

   
   


2. 
   

   If possible an explanation as to what is going wrong here.

   
   


3. 
   

   I am wondering if I could also call specific functions with the :: as I have seen it around but never used it because I don't know how it works.

   
   

Thank you