Nip them in the bud

This fellow who had spent his whole life in the desert comes to visit a friend. He'd never seen a train or the tracks they run on.

While standing in the middle of the railroad tracks one day, he hears the whistle, "Whooee Whoee!" but doesn't know what it is. Predictably, he's hit -- but only a glancing blow -- and is thrown, 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 one evening. While in the kitchen, he suddenly hears the tea kettle whistling. He grabs a baseball bat from the nearby closet and proceeds to batter and bash the tea kettle into an unrecognizable lump of metal.

His friend, hearing the ruckus, rushes into the kitchen, sees what's happened, and asks the man from the desert, "Why did you ruin my good tea kettle?"

The desert man replies, "Man, you gotta kill these things when they're small!"