Obama was in Des Moines, Iowa, kicking off the celebratory swing for "Let's Move," her signature project to reduce childhood obesity by promoting healthier eating and a more active lifestyle.
Before the first lady spoke, several children offered testimonials about their plans to improve their eating habits.
"We're going to drink milk!" said one boy.
"We're going to eat fruits and vegetables!" said another.
Gov. Terry Branstad proclaimed Thursday "Let's Move Day" in Iowa, which set a goal of being the healthiest state in the nation by 2016, as measured by a Gallup poll. It's now 16th.
"This is so cool!" Obama said. "Happy Birthday, Let's Move!"
Obama praised Iowa for being on the front lines of the healthier lifestyle movement.
"I'm proud that you guys are eating your fruits and vegetables," she said, "I'm proud that you guys are walking to school instead of taking the bus.
"We want every state in the country to do exactly what Iowa is doing," she said.
Even the cake to mark the second birthday of "Let's Move" was healthy -- a mix of oranges, bananas and other fruit.
After a rendition of "Happy Birthday," confetti and balloons fell from the rafters and showered the crowd.