Calculate the number 399

A man who had spent his whole life in the desert visited a friend. He'd never seen a train or the tracks they run on. While standing in the middle of the RR tracks, he heard a whistle, but didn't know what it was. Predictably, he's hit and is thrown, ass-over-tea-kettle, to the side of the tracks, with some minor internal injuries, a few broken bones, and some bruises.
After weeks in the hospital recovering, he's at his friend's house attending a party. While in the kitchen, he suddenly hears the teakettle whistling. He grabs a baseball bat from the nearby closet and proceeds to batter and bash the teakettle into an unrecognizable lump of metal. His friend, hearing the ruckus, rushes into the kitchen, sees what's happened and asks the desert man, "Why'd you ruin my good tea kettle?"
The desert man replies, "Man, you gotta kill these things when they're small."