There is a crack in my windshield, how that happen?

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Today afternoon, while I took my lunch break, I sit in my car and browsing my cell phone. One moment I noticed that on my windshield, there is crack line go from left to right at the bottom. I get off my car and take a closer look from outside, it was a real crack. Oh my God, how is that happen? Maybe one of those tiny rock flying on the highway hit on the windshield and I didn't notice it instantly, or something else I really don't know. Oh my God, I have to call my insurance again to ask if they would cover a crack windshield for replacement.