Investors looking to play the Chinese market must first do their homework, Jim Cramer said Friday on CNBC's "Stop Trading!" segment. Cramer said the strength of the world's fastest-growing big economy makes investing in so-called Red Chips -- the most-liquid stocks traded in China -- seem like a no-brainer to novice investors. But he said that that point of view, together with rising exchange-traded fund action tied to China, makes investing in these shares riskier, not safer. Cramer pointed to the iShares FTSE/Xinhua China 25 ( FXI) ETF, which comprises the 25 largest and most-liquid Chinese stocks. Cramer noted that it rose 79% for 2006 -- but is down 10% in the New Year. Big component PetroChina ( PTR) is down 9% off last week's new high. Cramer said the Chinese market offers great opportunity but that timing is key and that investors should let these names come in a bit before jumping in. Cramer said Friday's downgrade of Exxon ( XOM), the company that did nothing during the run-up and only started to rally after the downturn, shows that there is no hiding even for those companies that played it safe and bought back stock.