In some cases, the answer is very simple: It takes longer to open envelopes.
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When you vote in person, your ballot usually goes straight into the machine, being processed and counted on the spot. But with mail-in ballots, a few simple (but surprisingly time consuming) steps are added to the process. Someone has to open the envelope and flatten out your ballot. Someone also has to verify your signature matches the one on file.
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All of that is routine, but just takes more time. And this election cycle, a record-breaking number of ballots were cast by mail.
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In Pennsylvania, things are delayed even more because the state wasn't allowed to start the process of verifying and counting ballots until Election Day. Pennsylvania is the state with the most votes left to count, as of Thursday.
Some states, like Nevada, are also continuing to accept any mail-in ballots that arrive by Tuesday, Nov. 10, as long as they were postmarked by Election Day. That means votes are still trickling in, and all of those will be counted. However, Clark County, Nevada's most populous county, believes the "bulk" of all votes will be counted by the end of the weekend.
Remember, just because things take time doesn't mean anything is awry. Election officials have been asking for patience before the vote counting even started. They're taking their time to make sure everything is done properly, they say.
Despite President Donald Trump's request to "STOP THE COUNT!" on Twitter Thursday morning, elections officials nationwide have vowed they will continue to count every eligible vote that has been cast.