treasury bills

Money market mutual funds are known for being both liquid and solid. Liquid because you can get your money out of them at any time; solid because they maintain a steady value of $1 per share.

And they're popular because they tend to pay slightly higher rates than Treasury bills or bank savings accounts.

Individuals and institutions alike use them to park cash they are waiting to deploy and to ride out market storms.

Amid the turmoil of the past year, assets have soared 45%, to $3.5 trillion.

Wall Street had one of its best days in years Tuesday, with the major stock indexes rising sharply on news that the Federal Reserve is taking new steps to address the mortgage crisis.

The Fed said it would auction off $200 billion worth of Treasury bills to banks and brokerage firms and it would allow them to use their bad mortgage debt as collateral.

The Fed is hoping that by making more money available to banks, they'll lend more money to each other and to consumers.