Beware of thin markets. Thin markets are like thin ice. It's really hard to know whether you are skating on it or not. Take

Texas Instruments

(TXN) - Get Report

, which looks like it is ramping along with the rest of the semis.

Judging from the action, I thought there was a size buyer in the name. I'd bought a ton of stock on Friday at 140, and I wanted to scalp 5,000 shares when the stock was up almost a buck.

Bam! I go to sell my 5,000, and I knock the stock down more than a buck! Five thousand measly shares! Now that's a paper-thin market!

Of course, this a double-edged sword. If you have two buyers of anything today, it will take off to the stratosphere with very little push.

Thin markets are a constant problem in July. You don't feel the thinness if you are trading in hundreds of shares, but be careful out there if you are trading in anything larger.

James J. Cramer is manager of a hedge fund and co-founder of TheStreet.com. At time of publication, his fund was long Texas Instruments. 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.