Let’s imagine that in 1992, just after Nevermind peaked, Dave Grohl took a break from Nirvana to form Foo Fighters. I mean, why not? Grohl was a gun for hire, at least the sixth drummer to sit in with the band before they finally broke big. And let’s say he scored some of those juicy Foos radio hits right away: “This Is a Call,” “Big Me,” maybe “Monkey Wrench” too.

And then imagine he came back in ’93 to Nirvana in time for In Utero, making them even bigger than they already were—not just reliable album-sellers but the kind of band able to score regular Top 40 radio hits. Grohl would be transformed, from Kurt Cobain’s potent-but-silent sidekick, to coequal band focal point.

It’s a little hard to imagine for all sorts of reasons, not least the fact that Grohl was too respectful of Cobain to form his own project until both Kurt and the band were dead and gone. But the scheduling is also fanciful—who has that kind of time, to get a successful solo career going while keeping up with a best-selling group?

The fact is, it’s exceedingly rare for a successful side project to not only coexist with the original group but bring that stalwart act to new pop-chart heights. In fact, in chart history, it’s only happened three times (really, more like two and a half).

The third of these three acts is this week sitting atop Billboard’s Hot 100, in the form of the Black Eyed Peas*. “Boom Boom Pow” is, oddly, the act’s first No. 1—but it’s gun-for-hire Fergie’s fourth. More »