Another frequent reason for purchasing coins is as an investment. As with stamps, precious metals or other commodities, coin prices are periodical based on supply and demand. Prices drop for coins that are not in long-term demand, and increase along with a coin's perceived or intrinsic value. Investors buy with the expectation that the value of their purchase will increase over the long term. As with all types of investment, the principle of "caveat emptor" applies and study is recommended before buying. Likewise, as with most collectibles, a coin collection does not produce income until it is sold, and may even incur costs (for example, the cost of safe deposit box storage) in the interim.