Instructions
Grasp the mouse firmly by the tail with one hand. You should only do this with mice that are more than three or four weeks of age: If they're any younger, you might pull the tail off.
Pinch the base of the mouse's head between the thumb and forefinger of your other hand.
Pull each end of the mouse in opposite directions.
Listen for the crack and snap that indicates the mouse's neck has been broken. The mouse will probably go very still. Its legs may twitch reflexively, but this is just a nerve reflex. The mouse is already dead.
How to Kill Mice for Snake Food
Snakes may seem to be the ultimate predator, but if a live mouse is placed in with a snake--supposedly as food for the snake--the mouse might actually harm the snake, inflicting bites that scar or debilitate the reptile. Many snake owners pre-kill their feeder mice for this reason, giving the snake the benefit of fresh prey without any of the risk. This topic is the source of great controversy--snake owners agonize over whether there is in fact a humane way to kill mice for snake food, with each owner eventually arriving at the method that he, personally, believes to be the least traumatic to the mouse.