Holocaust Denial

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I remember way back in the day my grandfather would write letters to the editor regarding WWII coverage (Remembernce articles and such i guess). He was born in germany moved here and joined our army and went back over to germany and fought for us. every now and then crazy psycho people would look him up in the phone book and call and say the holocaust never happened stop spreading lies etc. These people would claim to be history teachers and all other kinds of stuff. Now my grandfather was as conservative as people come, and he knows it happened, he saw it. So how do these people really think it never happened? Is anyone here informed enough to debate for them? I doubt we have anyone this stupid on here. I cant even fathom how someone can think this. Even the reasons Wikipedia had, seemed rather outlandish. Either way, i just want to know how anyone other then complete and total nazi/moron could belive this.