WATER !

The U.S. government advises each household to have a 2 week supply of water. That is one gallon per day per person in the family.

Here is a very simple directive regarding water during an emergency from
www.ready.gov The follwing post (Water Purification) is more detailed.

"Always use bottled water for drinking and cooking if it is available. Unsanitary water can be very dangerous to your health. If you don't know the level of purity of the water, it should always be treated.
Use one or a combination of these treatments:
Boil
Boiling is the safest method of treating water. Bring water to a rolling boil for 3-5 minutes, keeping in mind that some water will evaporate. Let the water cool before drinking.
Chlorinate
You can use household liquid bleach to kill microorganisms. Use only regular household liquid bleach that contains 5.25 percent sodium hypochlorite. Do not use scented bleaches, color-safe bleaches or bleaches with added cleaners. Add 16 drops of bleach per gallon of water stir and let stand for 30 minutes. If the water does not have a slight bleach odor, repeat the dosage and let stand another 15 minutes.
Flood water can also be contaminated by toxic chemicals. Do NOT try to treat flood water.
http://www.ready.gov/america/npm08/tips/091908.html