As to bubble theory: With stocks, the total supply of shares is always held by someone, and the total quantity doesn't often change very quickly. The more recent buyers and sellers determine the more recent prices, and as preferences change, those prices can shoot upwards. Momentum players add to that and make a bubble. The dot-com bubble is the most recent example.
With houses, much the same situation is in play. The supply of houses is long term, they are not "consumed". People's preference changes and they pay more or less, and bubbles can ensue.
With goods and services that are produced and consumed over a short period of time, such as oil, this is NOT the case. Neglecting futures contracts for the moment, the actual good is transferred and used up. If the price is too high, oil supplies in storage and even on shipping tankers in the ocean start filling up with oil, because no one is paying the "market clearing price." (google that and learn) If the price is too low, the market clears and then shortages develop, and people end up lining up at gas stations hoping there is enough for everybody today. Such markets do not develop bubbles to the same extent. The futures market, due to it's nature, can create a temporary bubble, but it is always short lived as the contracts eventually expire and actual delivery and use is demanded. So the very short-lived price is strongly effected by things such as rumors of war, but the trend over several months is affected almost exclusively by supply and demand, not bubble think.
Interestingly, while this is true for oil, it is much less true for gold. Gold is not "used up" and can experience bubbles.
In summary:
Bubbles are possible with: Houses, Stocks, Gold
Bubbles are much less possible with: Oil, Food
When investing in the latter (oil and food), use supply and demand theory, which is fairly simple and reliable. Watch the stockpiles (are the stocks growing? if so the price will come down. Are the stockpiles shrinking? If so the price will go up).
When investing in the former (houses, stocks, gold) you will find prices much less predictable; bubbles will occur; people will change their minds constantly. Thus, invest in the former.
