"What’s funny about it is I actually thought it would flop," Eilish says of her first career Hot 100 chart-topper.

For the first time since early April -- back before the final season of Game of Thrones premiered, and when Woodstock 50 still looked like a possibility -- there's a new No. 1 song on the Billboard Hot 100.

Ending Lil Nas X's record-setting 19-week run atop the chart is Billie Eilish's "Bad Guy," which for a time appeared destined to linger at No. 2 for just as long following its March release. Back in mid-July, even a remix featuring Justin Bieber couldn't pull the song past the mighty "Old Town Road." But "Bad Guy" endured, giving Eilish the first Hot 100 No. 1 of her career.

While chatting yesterday with Eilish and her brother Finneas (her sole co-writer and producer) for a forthcoming interview, Billboard was excited to learn how they reacted to Monday's news, and why they think such a experimental pop song became so commercially successful.

What was your initial reaction when you heard "Bad Guy" finally reached No. 1?

Billie Eilish: My first reaction was I didn’t even care at all because, because -- listen here -- I was told [previously] we were trying hard to make it No. 1, and then I was told that it didn’t get to No. 1. So then I thought that it was done for, over with, [Lil Nas X] won. And then they were like, "It’s No. 1!" And I was like, “You’re lying.” Because I literally thought they were lying. [I figured] let [Lil Nas X] have his moment, but I was really happy to hear it actually was true. It’s been really great.

Finneas: I was deep in the final rounds of production on [a new project] when we found out "Bad Guy" was No. 1. I was super stressed out I was making the right choices on this new project... I think that’s the way to live your life -- to keep going. Keep making stuff. Obviously I'm very grateful "Bad Guy" is doing so well -- it's shocking and surprising and gratifying -- but I do think it’s important to try to make the next song that people are gonna be excited about.

"Bad Guy" was No. 2 for a combined nine weeks behind "Old Town Road." What was it like having such a big personal moment at the same time as something as massive as "Old Town Road"?

Eilish: It’s crazy to think about "Bad Guy" as a well-known song [Laughs].... Nobody doesn’t know ["Old Town Road"], you know? Having my song be above that song right now [on the Hot 100] is shocking to me. I think Lil Nas is so sweet; I met him a couple weeks ago. It’s so not a competition, it’s just funny … I don’t really care [about numbers]. It’s just a fun thing that happens in careers. It doesn’t really matter, but it’s kind of interesting, you know?

Since you never expected this particular song to be a hit, this all must be crazy for you now.

Eilish: What’s funny about it is I actually thought it would flop… because the chorus doesn’t have a hook -- at least what we wrote as the chorus. Because the chorus is [hums synth riff of "Bad Guy"]. My brother and I actually sat in his room trying to write something else over that but we couldn’t do it because nothing else worked. [We didn't] want it to be empty there, but we were just like, "I don’t know if anyone’s going to like this, because there’s no lyrics here." For some reason people do like it and it’s huge now. It’s one of my favorite songs I’ve ever made. It’s my favorite to perform, it’s so fun. I’m so proud of that song.

From your perspective -- being online, interacting with fans, performing it -- why do you think "Bad Guy" has resonated with so many people?

Eilish: I think because it’s fun. It’s a weird thing to hear come out of a girl’s mouth. I think Nicki Minaj said in one of her songs ["Chun-Li"] that she’s the bad guy, and I didn’t realize that until recently. It’s such a cool juxtaposition of that sentence. Why do people resonate with it? I know why I resonate with it, but they’re all my own reasons. I think it’s just fun. It’s catchy. It’s a weird-ass song! But it’s catchy.