The word bubble is sometimes used to mean a time of unexpected, or unearned prosperity or happiness. Like a real bubble, the metaphorical bubble is expected to be short-lived and easily broken.
This metaphor is especially apt when used to describe the performance of certain stocks on the stock market. When investors were excitedly buying "dot-com" stocks in the 1990s, the stock prices rose dramatically, far beyond the actual value of the companies. This bubble, like a bubblegum bubble, couldn't expand forever, and it didn't.
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