How to use Makefile from a very simple example [closed]

I'm trying to learn how Makefile works, so I decided to try it with a very simple code. This is what I wrote:

/* justify.c */

#include <stdio.h>
#include "line.h"
#include "word.h"

int main(void) {
    printf("I'm in the main.\n");
    read_word();
    write_line();
}

/* line.h */

void write_line(void);

/* line.c */

#include <stdio.h>
#include "line.h"

void write_line(void) {
    printf("write_line\n");
}

/* word.h */

void read_word(void);

/* word.c */

#include <stdio.h>
#include "word.h"

void read_word(void) {
    printf("read_word\n");
}

now ... if I do everything from the terminal it works:

> gcc -c justify.c
> gcc -c line.c
> gcc -c word.c
> gcc -o justify justify.c line.c word.c

but if I try to do everything with a Makefile it gives me an error:

# Makefile 

justify: justify.o line.o word.o
    gcc -o justify.o line.o word.o

justify.o: justify.c line.h word.h
    gcc -c justify.c

line.o: line.c line.h
    gcc -c line.c

word.o: word.c word.h
    gcc -c word.c

> make justify

Undefined symbols for architecture x86_64:
  "_main", referenced from:
     implicit entry/start for main executable
ld: symbol(s) not found for architecture x86_64
clang: error: linker command failed with exit code 1 (use -v to see invocation)