BOULDER, Colo. — During an unscheduled stop at a restaurant and bar here late Tuesday, President Obama ordered a pizza, shook hands, posed for pictures, bought some t-shirts — and got some yogurt spilled on his pants by an enthusiastic fan.

President Obama cleans yogurt from his pants. (BRENDAN SMIALOWSKI/AFP/GETTY IMAGES)

The woman replied, "I'm very embarrassed."

At the restaurant, called The Sink, one group of students was drinking Pabst Blue Ribbon beer and posed for a picture with the president.

Obama spent about 20 minutes at the restaurant and then left for his official event, at which he continued to push for Congress to take action to prevent student loan interest rates fromdoubling, as they are scheduled to do in July.

Inside the very hot Coors Events Center at the University of Colorado, supporters had been waiting for hours, fanning themselves and fetching cups of water from a bank of portable water fountains.

"All of you who have seats, feel free to sit down because it's hot in here, and I don't want folks dropping off," Obama said after taking the stage to thunderous applause and cheering.