> We get this one a lot - how did it come to be that people yelled "Geronimo!"
> when they jumped from a place?

The story I remember reading as a kid:

Early American Paratroopers (early days of WWII) in training were instructed to call out their names as they exited the door of the aircraft. One of the early troopers was a Native American, who started yelling "Geronimo," the famous war chief of the Apache, rather than his own name. This caught on with other troopers in his unit and spread from there.

However the truth is more like this:

Prior to America's entry into WWII, the Army first experemented with the airborne forces concept at Fort Benning, where a parachute test platoon was established. One day, four off duty members of the test platoon saw a movie at the Post theater that feature Calvalry chasing Geronimo and the Apaches. After the film one of the novice troopers started kidding another, a Private Aubrey Eberhardt about being scared of the next day's scheduled jump. Eberhardt got indignant and told the others that the next day, to prove he wasn't scared, he would shout "Geronimo" at the top of is lungs when he went out the door. To calm their buddy, the other three agreed that Geronimo sounded like an appropriate jumping cry.

The next day, when Eberhardt jumped, his shout of "Geranimo" could be heard from the ground. The cry quickly spread to the whole test platoon, and then as the airborne force expanded, through the force. Later in WWII , "Geronimo" gave way to the Army's insistance that a trooper count out loud (one thousand, two thousand... ) waiting for the main chute to open (if he gets to three thousand and the chute hadn't opened he was supposed to use his reserve). Still some troopers would bend the rules and shout "Geronimo" before starting to count.

Source: Gerald Devlin "PARATROOPER", 1979, St Martin's Press.