Cramer explained that in the old days, if a company did well its stock was rewarded; if it didn't, its stock fell. But in today's global market, with ETFs, futures and high frequency trading, this is simply no longer the case. Cramer said that makes understanding why the your stocks are falling crucial. Cramer said in a really tough market it seems like all stocks are trading in lock step, with the good, bad and the ugly all heading lower. Why is this the case? Cramer said it's because ETFs and hedge funds, along with the futures markets, have turned stocks into commodities, baskets that can be traded at will, despite the underlying fundamentals. But, Cramer noted, when the selling is done and the panic is over, the fundamentals begin to matter again, which is why stocks with great fundamentals are important. He said the "paper risk," or risk that stocks can go down for any reason, is always present in today's market, and investors need to be aware. Cramer also warned against risks from short-sellers. He said when the Securities and Exchange Commission discontinued the uptick rule in 2007, it allowed short-sellers to gang up on stocks like never before. He said short-sellers have contributed to the failure of banks in 2008, and are at it again, selling European bonds. Then there are also the double- and triple-levered ETFs, the ones Cramer has railed against many times in the past. He said these funds serve no purpose but to allow big money to make quick gains at the expense of the rest of the market. He said funds like the UltraShort Financials ProShares ( SKF), have cost uninformed investors big money because they don't work as advertised. Cramer said his bottom line is that stocks are not cash, and they don't act that way. Investors need to be aware of market risks, paper risks and the risks of short sellers, before the invest their nest eggs.