OAKLAND -- We finally have an answer.

It came during Nickel Creek's sold-out show Monday at the Fox Theater, when mandolinist extraordinaire Chris Thile decided to end all speculation and reveal exactly why the band originally parted ways back in 2007.

It wasn't due to one of the standard breakup reasons -- such as the dreaded "creative differences." It was something far more dire than that.

"It's because we ran out of instrumental titles," Thile told the capacity crowd in Oakland.

Once they were able to find some more names, Thile continued, it was time to get the band back together.

OK, so he was kidding -- I think.

The point is that Nickel Creek is back in action, after a nearly seven-year hiatus. And the reunion coincides with the band's 25th anniversary.

Fans were certainly glad to witness the return. They wholeheartedly cheered the group throughout the approximately two-hour show, embracing both old and new tunes with equal joy.

The progressive bluegrass act, which also features Sara Watkins on fiddle and brother Sean Watkins on guitar, sounded as good as ever as it raced through nearly two dozen acoustic songs. The trio's sound was fleshed out by some expert bass work by virtuoso talent Mark Schatz, who also added some dance steps and a hambone routine to the mix.

One of the things that made this reunion so special is that it was beginning to look like it might never happen. Seven years is a mighty long break, especially for an outfit that was formed by a bunch of preteens. (Sean, the band's oldest member, was just 12 when Nickel Creek started.) Plus, all of the players went on to experience success outside of the Creek -- especially Thile, who collaborated with such instrumental titans as Yo-Yo Ma and Edgar Meyer.

No wonder fans quickly snatched up all the tickets for the Fox show. The reunion ranks as one of the true music events of the year.

The Southern California group, which first came to widespread fame with its platinum-selling eponymous effort in 2000, is offering more than just nostalgia on this trek. It's also pedaling a long-awaited new album, the recently released "A Dotted Line," which is Nickel Creek's first outing since 2005's ominously titled "Why Should the Fire Die?"

The trio seemed eager to showcase the "Dotted Line" material, opening the show in solid fashion with "Rest of My Life." Three songs later, Nickel Creek offered up another new song, "Destination," and then continued to sprinkle "Dotted Line" tracks throughout the show.

The group's sound hasn't changed much over the years. It still melds pop and traditional styles into what might be best described as bubble gum bluegrass. It's not Ralph Stanley or Del McCoury, by any means. Yet, it could certainly lead listeners to those elder statesmen of the genre.

The instrumentals were the most impressive offerings of the night, mainly because they gave ample opportunity for these four players (and I'm including Schatz in that number) to shine. They are all superb instrumentalists, yet Thile is in a class by himself.

He's one of the greatest mandolinists in history, yet he doesn't attempt to prove it on every song. He shows a great deal of restraint, obviously understanding that fireworks are more effective when used only on special occasions. To use a guitarist analogy, he's more like the always tasteful Mike Campbell (of Tom Petty and the Heartbreakers fame) than the flashy Eddie Van Halen.

Nickel Creek brought the show to a close in fine fashion, again highlighting the new album, with such tracks as "Where Is Love Now" and "Hayloft." What that tells me is that this reunion has a real chance at stretching far beyond this 25th anniversary.

The band returns to the Bay Area this summer, to perform an Aug. 4 concert at the Mountain Winery in Saratoga. Tickets are $39.50-$69.50, www.mountainwinery.com.

Follow Jim Harrington at http://twitter.com/jimthecritic, and www.facebook.com/jim.bayareanews.