Greg is smooth at explaining these things.
Hopefully this explanation will help limit your concern regarding bubbles. The great thing about investing in stocks, is that the companies are public (owned by shareholders) and must report their financials information each qtr. When stocks start to get bubblicious, you can typically see this in their price to earnings (PE) ratio. Since 1900 the average PE ratio of the S&P 500 is ~15. The current PE ratio of the S&P 500 is around 13, which would suggest that the S&P 500 is not overpriced on a historical basis.
For educational purposes I'll compare 2 growth stocks that do not pay dividends to show you how to look for potential bubbles. First, let's calculate Apple's (AAPL) PE ratio. Apple's stock is currently trading around $403 per share. Wall street is estimating Apple's 2012 earnings at $34.77 per share (EPS). Divide the stock price by the EPS ($403/$34.77) and you get a forward looking PE ratio of 11.6. Which is lower than the S&P average. Now lets look at Amazon (AMZN), they are currently trading at $173 per share and wall street is expecting them to earn $2.01 per share in 2012. Their forward looking PE ratio is 86, which to me is an extremely high PE ratio. So getting back to the bubble comment, when looking at investments for Investing 102, look for stocks with a reasonable dividend yield, that has a history of paying and growing their dividend AND has reasonable PE ratio.
Hope this helps!
Rob