What's It to You, Anyway?

Yahoo! joins the S&P 500 tonight. Cramer explains what this means for investors.
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Yahoo!

(YHOO)

defines this day. It goes into the

S&P 500

tonight. That means funds will have to sell other stocks to get into Yahoo! as the stock Yahoo! is replacing,

Laidlaw

(LDW)

, is weighted much less in the index.

Join the discussion on

Cramer's Latest.

Yahoo! Board

What does it mean for you? Historically, with these highfliers that have a concentrated float, that means it keeps going up until the end of the day and then sells off tomorrow or tonight, given that trading in Yahoo! never stops.

That's what happened with

America Online

(AOL)

. It is also what happened with

Microsoft

(MSFT) - Get Report

.

The rest of the S&P should be going down all day as these index funds raise the cash to make that purchase. Twenty million shares of Yahoo! has to be bought as part of this process. (At least that's the estimate; no one has an exact number.)

OK, let's cut to the chase. What am I doing? I am selling some with the idea that I can buy it back tomorrow -- lower.

If you are a long-term guy and you believe in Yahoo!, hit the next button. If you are a short-term guy and you believe in Yahoo!, well, that's me. And if you don't believe in Yahoo!, I guess this is your day to short it!

James J. Cramer is manager of a hedge fund and co-founder of TheStreet.com. At time of publication, his fund was long Yahoo!, America Online and Microsoft. His fund often buys and sells securities that are the subject of his columns, both before and after the columns are published, and the positions that his fund takes may change at any time. Under no circumstances does the information in this column represent a recommendation to buy or sell stocks. Cramer's writings provide insights into the dynamics of money management and are not a solicitation for transactions. While he cannot provide investment advice or recommendations, he invites you to comment on his column at

jjcletters@thestreet.com.