setuid doesn't work in /tmp

Question

I wrote a C program, expecting to see the difference between uid and euid with setuid bit enabled:

#include <unistd.h>
#include <stdio.h>
int
main(void) {
    printf("euid: %d\n", geteuid());
    printf("uid: %d\n", getuid());
    return 0;
}

Then I compiled them in my home directory and /tmp directory respectively but got different results. The script I used to compile and run the two programs:

#!/bin/bash
echo in current directory
sudo gcc -o a.out a.c
sudo chmod u+s a.out
./a.out
echo 
echo in /tmp
sudo gcc -o /tmp/a.out a.c
sudo chmod u+s /tmp/a.out
/tmp/a.out

The result of the executation:

in current directory
euid: 0
uid: 1000
in /tmp
euid: 1000
uid: 1000

The setuid bit didn't work in /tmp! I wonder why.

I mv the /tmp/a.out to my home directory and the setuid bit worked, so I guess there is something to do with the /tmp directory?