Water
Storing water is critical to surviving even the smallest crisis. A human being will die if they go without water for as little as three days. The U.S. government and other emergency relief organizations advise us to keep a minimum of one gallon per person per day on hand. To cover a three day emergency for four people and one dog, you would need a minimum of 15 gallons of water stored.
The one gallon per person per day is the minimum needed to cover basic hydration and limited cooking needs. If you ration the water well enough you may have small amounts left for minimal hygiene requirements such as rinsing your hands. The amount of water each person drinks will vary based on weather conditions and health. Hot or windy days will dry you out faster and you'll need to drink more water. Salty foods will also dehydrate you, so keep this in mind if you store a lot of canned goods. Pregnant or nursing women and anyone with certain medical conditions will require more water as well.
How you choose to store your water depends a lot on personal preferences, convenience and space. Most families find it easy to simply buy bottled water in individual drinking bottles, and stack the cases in a closet or out of the way space. Alternatively you can wash used soda and juice bottles, then fill them with clean water for storage. Do not use empty milk containers however, because the material those are made with can leach milk byproducts back into your stored water.
Another option is to purchase empty water jugs in one, five or seven gallon sizes, then fill them from your own tap and store them out of the way in various corners of your house. Larger families who have the storage space might consider buying a food grade 55 gallon barrel and filling it from the tap. Fifty-five gallons of water could sustain up to 18 people or animals for three days, or give a family of five or six people plenty of wiggle room for cooking, cleaning and hygiene.
Like food, it helps to keep water rotated so it stays fresh. If possible, empty and clean your water storage containers every six to 12 months, and refill with fresh water. As long as the water is stored away from sunlight however -- because sunlight encourages algae and bacteria growth -- you can generally use water that has been stored for a long time. To be on the safe side, you may want to consider learning how to sterilize the water with a few drops of bleach, or purchase a water filter to use in the case an emergency. If you have the ability to cook during the emergency however, you can easily sterilize your drinking water by boiling it for a full 10 minutes.