I would think a bubble requires something that can be bought and sold (gold bars, equity, tulip bulbs, condos in Shanghai). You buy it with the expectation that

I would think a bubble requires something that can be bought and sold (gold bars, equity, tulip bulbs, condos in Shanghai). You buy it with the expectation that you can resell it for a higher price. Or, more to the point, you accumulate and hoard beyond what is sane or reasonable in the expectation of higher prices. For any given individual its difficult or impossible to say what the real value of an education is going to be in the future. Even if there are no prospects for someone with a degree in linguistics today, it doesn't mean that the degree doesn't open a door tomorrow, perhaps in a related field.