What is the quickest and most painless way to kill an animal?

My parents are kicking me out next week, and I have nowhere else to go. I have no problem with this because I am a good survivalist, and since I live near the countryside I can sleep wherever I want (if its safe and secluded of course). Now, I know two ways I can get food and two ways I can get water. There are lots of fairly clean rivers and ponds around the countryside, and I can pinch bottled water from the town shops. I can also pinch food from the shops, but if for some reason I can't do that (getting banned from the shops, shops closed, etc) I plan to get meat and blood from the animals on the many farms tered across the countryside. I don't want to see them suffer too much... So what is the best way to kill one quickly?