Ok, how and why does money 'grow'? If you put it in a bank then the bank is willing to give you more money ("interest" they call it, for some strange reason). But they don't do that because they're generous. They do it because they believe they can use your money to make more money. Enough so they can pay some to you. So - they "invest" it. Which just means that they give it to a company that is making something (like a car) or doing something (like washing cars) and has demonstrated that they can do more of it (and so make more money) if they had more money to hire more people and supplies.

Great. So far so good. Everyone is happy. But what if there's more money trying to find places to invest than there are good investment opportunities? Well, 2 things happen:

1. the good investment opportunities have lots of offers of money so they can get a better deal (ie, they can keep more of the profits themselves and give back less to their investors because they're considered 'a safe bet')

2. the more questionable investment opportunities begin to look more attractive. Sure, they're riskier (they might lose all your money or some of it) but they're willing to give you more money back if they succeed.

And so a bubble is formed when there is more money looking for ways to increase itself than there are good opportunities for that money. And that's not sustainable. It will end with people realizing they're making bad bets either because they lose their money or because they see many others losing theirs. So they pull their money out. And suddenly, the things that seemed valuable recently, are suddenly deeply devalued. And that's the bubble bursting.