Even if there this is an ever expanding bubble, some investors seem happy to jump in.

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For instance, the dot-com bubble eventually reached a point where day-trading retail investors became a dominant force, creating incentives for venture capitalists, underwriters and fund managers to jump on the bandwagon, hoping to hop off before the bubble burst.

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But it is impossible to know for sure whether a jump in share prices reflects an increase in productivity growth or a bubble.

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Investors who are sensing high stock returns and are planning to jump back into the market must be careful and remember that this could be yet another bubble followed by yet another crash.

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If the market is willing to enthusiastically jump on the Groupon deal, then maybe we really are in some kind of weird, narrowly defined bubble.

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