The NAV of a money market fund normally stays constant at $1 because investment products usually do not produce capital gains or losses. As such, the principal in a money market fund usually remains constant, making risk exposure non-existent compared to stocks, bonds and non-money market mutual funds.
The first case of a money market fund breaking the buck occurred in 1994, when Community Bankers U.S. Government Money Market Fund was liquidated at 94 cents because of large losses in derivatives.