Essential Elements of Friendly Information (EEFI) are critical aspects of a friendly operation that, if known by the enemy, would subsequently compromise, lead to failure, or limit success of the operation, and so must be protected from enemy detection.
The location of the water balloons was an EEFI to operation "Soak Suzie." Hobbes failed to watch his EEFIs and divulged where Calvin stored the water balloons during a tummy rub. The mission resulted in utter failure.
by AZ Boomer May 20, 2013