Welcome to Election Day. There are, essentially, a billion races happening, and it's hard for any mere mortal to keep track of it all.

But there are also two big, simple questions: Who will control the House? Who will control the Senate?

Let us give you the rundown on how we're approaching the night, and when exactly we'll know if this is the wave election Democrats hope for and Republicans fear.

Most calls will likely be made between 9:30 p.m. and 11:30 p.m. ET. After that, we'll be waiting on calls that are probably a) really close races, b) races from parts of the Midwest and the West Coast.

It’s pretty likely we’ll know by 9 p.m. or 9:30 p.m. ET if we’re looking at a major wave election or not, based on data coming from the East Coast and Midwest...

...but it’s worth playing around a little with this map, as you’ll get a sense for how tricky even the House landscape is. Democrats need to flip 23 seats to take control of the House. That's actually a lot, and it's definitely possible that Democrats could win in many places and have a strong night on Tuesday, without taking control of the House.

You'll know it's a huge wave if: It's 9 p.m. ET and people on Twitter and on cable news (and especially on the AM to DM Election Night Special, where you'll find me and Decision Desk HQ's Brandon Finnigan) are talking about massive Democratic turnout along the East Coast. Democrats are already winning House seats in states like Virginia, Pennsylvania, New Jersey, and Florida, and maybe in places like Ohio, Kentucky, and Texas. Andrew Gillum and Joe Donnelly are winning decisively in Florida and Indiana; Stacey Abrams is coming close to 50% in Georgia; and Phil Bredesen is doing pretty well. And, critically, the early returns in the Midwest look incredibly strong for Democrats.

You'll know it's not a wave if: It's 9 p.m. ET and you're seeing a lot of tight races along the East Coast, and maybe some where Republicans are holding on, especially in Pennsylvania, New Jersey, and the Midwest.

One to watch is New Hampshire's 1st District: For the last 10 years, its vote total has been within 1% of the national House vote. Returns start coming in during the 7 p.m. ET hour, so it starts pretty early, too. A Democrat and a Republican have been swapping this district back and forth for years, in some kind of New England Hatfields–McCoys thing. Currently, Democrat Carol Shea-Porter holds the seat, but she is retiring, leaving Republican Eddie Edwards vs. Democrat Chris Pappas.

The Democrats have different paths to taking the House, because many districts sort into a few groups: wealthy suburbs; places that voted for Trump and are more working-class; and more middle-class suburbs. More on some of those below, but it's not like we can just say, “Look at these 20 districts.” Different scenarios could happen.

How a race gets “called” (it really is this simple):

First, take a state like Vermont: We all know Bernie Sanders will win reelection tonight. Some news organizations will call that race the second the polls close; they're projecting that it will happen, but it's not like they objectively know it will. Others will wait for some Election Day data to confirm that voter patterns we know from previous elections still hold.

Basically for every race, that same process is happening among smart, fallible humans who work for news organizations — just with more data and caution. When a news outlet says, e.g., Donald Trump will win Florida even though all the votes aren't in, what they're looking at is the current voting results against their historical knowledge of counties within a state and how they vote, and making an informed judgment that the votes that still need to be counted aren't enough to change the outcome.

This year, there are three different groups providing unofficial results on election night: the Associated Press, Decision Desk HQ, and Edison Research. Each of those groups is using slightly different systems to get the data from election boards. They then feed that data to news organizations like BuzzFeed News, the New York Times, or CNN. The data works sort of like a fountain soda — you can pick Coke or Dr. Pepper, but you still get a full cup of soda either way. Different organizations may not be using the same data, though, which means the New York Times map may not look like the maps you see on BuzzFeed News. State agencies will officially certify results in the next days.

One of the scenarios in which foreign or domestic troublemakers try to disrupt the election involves interfering with or hacking one of these unofficial, media-led systems for calling the election, spreading confusion and doubt. Those multiple sets of data could help stop that.

Let's get on with this, starting with...

THE 6 P.M. ET HOUR