BOULDER, Colo. - President Obama's unscheduled stop tonight at The Sink Restaurant and Bar, a local watering hole on the University of Colorado campus, turned into a messy affair as he mingled with students and took photos outside.

A young woman spilled yogurt on the president just as he was wrapping up and about to get back inside his armored black Chevy Suburban.

The milky substance appeared to hit his pants, prompting the president to begin wiping himself off with a towel handed to him.

"Getting yogurt on the president, you've got a story to tell," Obama told the lady, according to a pool reporter on scene.

"I'm very embarrassed," she replied.

Obama later made light of the moment in his speech at the campus' Coors Event Center before a crowd of 10,800 students.

"A woman got very excited and spilled yogurt on me," Obama joked.  "Or hazardously, she spilled yogurt on the Secret Service. The agent just stood there, looking at her."