Last year, Kelly Clarkson celebrated the 10th anniversary of her winning American Idol.  But on Monday, she was surprised to be reminded of yet another milestone this week: the 10th anniversary of the release of her debut album, Thankful.

Thankful came out April 15, 2003, and when a fan mentioned that on Twitter, Kelly replied, "YEAH!!! So weird it's been over 10 years....who knew?!"

Thankful debuted at #1 on the Billboard album chart and featured the top 10 hit "Miss Independent," co-written by Christina Aguilera.  It's since gone on to sell over four million copies worldwide, and helped establish Kelly as a viable artist, not just a graduate of a TV singing competition.  In the past ten years, Kelly has more than lived up to the promise of Thankful, scoring album sales of 23 million, as well as three Grammys, a string of hit singles and a worldwide fan base. She's also remained relatively scandal-free, while enjoying a reputation as one of the most gifted singers in pop music today.

Kelly is currently preparing for her summer tour with Maroon 5, which she previewed last week on American Idol while performing her new single "People Like Us."

She tweeted, "Thank you everyone for all of your sweet words about my new single :) I freakin love singing it! Planning my show right now for this fall when I tour with Maroon 5 and 'People Like Us' is one of my favorite parts of the show."  She added the hashtag "#bringglowstickspeople," indicating that she'll be repeating that glow-in-the-dark look she rocked on Idol on the road this summer.