And while we're on the subject, NEVER leave any tools near battery posts(Ed!). And ALWAYS disconnect the ground first and connect it up last.
Why? If a wrench slips or touches anything metal on that first or last connection(ground) nothing happens. But if you connect the positive side first or last, EVERYTHING metal can make sparks/weld/explode said battery. This is what I was taught by my Dad, and it has saved me numerous times.