Where do Stand-Up Comedians Get Their Material?

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Recently I was watching a stand-up act and I was wondering where on earth do they come up with this stuff, how can they do this day after day and always have more to say? Is it self-inspiation or are they paying someone to sit and think up these jokes? Most of them are always talking about personal experiences, but do you think they are really true or is is just an act? What's your opinion?