Andrew W.K. played piano at Hanson's big Supper Club show last night. That was strange enough. And Harlem's entire Young Love Choir sang. But no one who joined the blond brothers onstage was louder than the roomful of Hanson fans. This spring marks the tenth anniversary of the sibling-pop trio's single "MMMBop" (yes, they played it), and every guitar solo, song introduction, and minor gesture to the audience elicited a wave of Woo!s louder than the last. Despite the single-digit weather, fans camped out on the sidewalk early to get in, as evidenced by the Starbucks cups dotting the pavement. (See? Way more mature than the Mountain Dew they were drinking on line a decade ago.) But why did Andrew W.K. sign himself up for the popsters' comeback?

The "Party Til You Puke" author had collaborated with Hanson on their annual Fool's Banquet songwriting retreat, and he may have helped give them an edge on those singer-songwriters who score very special One Tree Hill episodes and even more special One Tree Hill commercials. "Go," the first single off Hanson's upcoming album, has the kind of soaring falsetto chorus W.K. would approve. And he might have taught the brothers a few rock-star maneuvers: When insistent feedback derailed the set, middle brother Taylor kicked over the offending speaker, calling the high-pitched whine their "Radiohead moment."

And, when you think about it, maybe Hanson and W.K. aren't such a weird pairing. The brothers are best known for a feel-good hit, while the punk icon entreats fans to start each day by kicking self-doubt in the face. These guys are all about optimism. And though W.K. refrained from fighting personal demons onstage, he did provide a frantic, skittering piano solo during an encore rendition of Cat Stevens's "Peace Train." Go humanity! —Lindsey Thomas