If the polls just closed, how can AP already declare a winner?

By Robert Yoon, Associated Press

WASHINGTON (AP) — Control of the White House and the narrowly divided Senate and House may hang in the balance on Nov. 5, but don’t expect election night to be all nail-biters and barn burners. Chances are some winners will be declared before a single vote has been tallied, just as the polls close in those states.

So what gives?

Although determining a winner before any votes have been counted may seem counterintuitive, race calls at poll closing...