Why does it take longer to count mail-in votes?
"There are usually two or three envelopes involved and signatures on different envelopes and marking, you know, sort of the kind of bars so that you can track that over time. So it takes longer because you're trying to open all of those things up," said Janine Parry, a professor of political science and Arkansas poll director at the University of Arkansas.
"Election judge has to make sure the signature's there, it looks like your signature. Then they open the ballot, and they start sort of making a pile to be fed into the counter," said David Hawkings, the editor in chief of The Fulcrum.
Processing and tabulating mail-in votes takes longer because all the verification that's normally done at a polling place still needs to happen. Plus mail-in ballots require extra steps, like opening the envelope and unfolding the ballot.
Some states are allowed to start processing mail-in ballots as soon as they arrive. Others can't do any verification or processing till Election Day.
"And given the fact that we're going to have so many mail ballots this year, if we have to wait a few days to have an accurate count, that's a good thing," said Bob Brandon, the president of the Fair Elections Center.