As someone who has eaten hardtack, I definitely recommend finding something else to eat. It is flavorless, it is difficult to render it edible, and you could use hardtack to tile your fucking garden path.
Canned goods, jerky, dried fruit, and MRE's are all readily available and long-lasting. You can also get freeze-dried food like Mountain House. Officially, most military rations have a maximum shelf-life of ten years, but they generally last much longer than that. I've eaten rations left over from Desert Storm and even Vietnam and suffered no ill effects.
What I suggest is get thee a nice couple of square meters of land (be it backyard, garden, whatever) and dig a hole, cover it in plastic, then put a layer of transparent plastic on it, with a hole right in the middle, put a rock heavy enough to make the plastic all funnel-like and... voila! Instant rainwater reservoir! Nothing is purer than rainwater!
First of all, this assumes you have a couple square meters of land.
Second, you do realize that rainwater is NOT pure, right? The entire process of formation of raindrops is moisture condensing on atmospheric dust particles until they get heavy enough to fall.
And that doesn't include contaminants like birds crapping on your rain gathering rig, leaves and dirt getting blown into it, etc.
If there's some sort of accident (massive fire at a factory or chemical plant) or war (chemical weapons, nuclear weapons), your rainwater is not only NOT pure, it's also likely to fucking kill you, or at least make you sick.
At the very least, when you gather rain water for drinking, you need to boil it before drinking it. Preferably you should be boiling and filtering it. If it's full of radioactive fallout or chemical contaminants, I wouldn't even bother with it. You can't even use it to bathe in. Just rely on bottled water, which is cheaply available in jugs and cases of bottles. You can also keep tap water in other containers (large soda bottles, with a tiny bit of bleach or iodine, and I do mean TINY) added, kept in a black garbage bag to prevent bacteria/algae growth). If you have a basement, 55 gallon drums are inexpensive and having two or three full of fresh water (cycle it so they're always full of fresh water, not stale) goes a long way toward keeping you set on drinking water.
In a worse case scenario, you can also fill the tub and use that for drinking and flushing the toilet. In a pinch, draining the water heater will provide you with a few gallons of water too. If there's an issue with contaminated water, beer will do in a pinch (as in New Orleans after the hurricane) so long as you remember that too much of it will dehydrate you rather than hydrate you, and hard liquor mixed with water will help kill bacteria in it. Again, boiling is recommended. Just be sure to boil it BEFORE putting alcohol in it.
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