This guide will give you some things to think about when considering a Time Capsule purchase.
A Time Capsule is a container that holds records, such as documents, photos and memorabilia, to be opened at some predetermined opening date in the future. A time capsule is a snapshot of today. It can be a way to go "back to the future," because it affects not only those who place it, but it also affects future persons who open it. In a Time Capsule, today's things become tomorrow's history. Eventually. But how do you choose one?

1. Perhaps the most important question is the first one: What is the reason you want the time capsule? You might use a time capsule to mark a special anniversary---whether your tenth wedding anniversary, your baby's first birthday, or your home town's centennial or sesquicentennial celebration. Then again, you may just want a time capsule for 'fun' or for a temporary learning project. Certainly, not every time capsule project needs to be a serious study in preservation! If you know your purpose, it will be an easy task to find the right time capsule.
2. Where you place your time capsule can be the determining factor in the type of capsule you end up buying. If you just intend to place the capsule on a shelf in your living area, for instance, almost any time capsule that does no harm to the contents will probably do just fine. You would not want to bury any of these time capsules (although if you had to do so, the stainless steel ones would be better than the fun tin kinds). You should also not place any time capsule in a place whose environment changes (such as an attic or a basement).
3. How long do you wish to allow the items in your time capsule to lie undisturbed? One question to ask yourself, "Do I want to be around to open this in the future?" Larger organizations will want to consider a more serious time capsule than the individual if the time capsule is planned to be opened in more than 25 years.
4. How will you make sure the time capsule will be remembered and not forgotten or lost? Often some kind of a metal plaque is placed over the cornerstone or area that encloses the capsule. A city's mayor can hand down instructions along with the  'keys to the office' to his successor as he leaves office. Another way is to register the serious time capsule with the International Time Capsule Society (one near Atlanta in the US and one in London in UK).
5. Perhaps what will most tickle your imagination is deciding what kinds of items will you place inside the time capsule. On one hand, future folks will experience a bit of what life might have been like now for us. On the other hand, we can have some fun trying to predict what life will be like in the future. Of course, you'll want to make your contents interesting enough so that future folks will look forward to opening it. Just as 'one man's junk is another man's treasure,' some of the more mundane things of today may turn out to be collectible in just 25 or 50 years. The term 'ephemera' means something meant to last only a short time because it was produced for a particular purpose and nothing more, because it was destined to be thrown away. Ask yourself how we can try to predict what tomorrow's collectibles will be. Rather than trying to make such a forecast, however, you may want to stay with the photos and memorabilia of your own family or group that you know will be good to save. A popular discussion is whether to save photos onto CD or DVD and whether, assuming the media lasts (a big enough assumption), one can find the media to play or edit it again. Suddenly, 25 years seems like 100.
6. How can you determine the best size for your time capsule? Many times, it depends on what sizes of things you want to save, and how you wish to save them. In general, the less you roll an item, the better, because when a document or photo is rolled for any length of time, it will have to be relaxed and flattened many years later.
7. What can happen to time capsule contents over time and why should you be concerned about it? One of the most popular time capsule ephemera items is a daily newspaper, yet this paper is full of acid. If stored bare with a photo, scroll or document, the acid will off-gas and pollute the air and therefore the more alkaline items inside. Today's scrap bookers are among those who are more aware of damaging papers and inks. You see deacidification sprays for sale that you can use to change the pH of papers from acidic to alkaline, and you can purchase permanent papers and archival pens, too. Anyone who has ever seen the deterioration from those so-called 'magnetic' photo albums of the 1970's will not question the problems with acidic glues and vinyl plastics being next to precious photos over time. As with anything, you need to ask yourself how important the papers and photos are to you - if they're damaged, how important is that to you?
8. What is the best time capsule container to use for your items? While it's easy for me to say 'stainless steel' - the answer really depends on how you've answered the above questions.