[lug] OT: cc question

[Stephen Queen]

If I enter the following simple program

#include <math.h>
#include <stdio.h>

int main()
{
        double results;

        results=sqrt(4);
        printf ("%f\n", results);

        return 0;
}

and compile it with the following on the command line

cc simple.c

I get the following error

/tmp/cc40XqaS.o(.text+0x1b): In function `main':
: undefined reference to `sqrt'
collect2: ld returned 1 exit status

If I then use the following command to compile it, it works correctly.
cc simple.c -O2

This is true on debian and slackware. Does anyone know of an explaination?