Nearly every dose of Workplace Fitness we have provided for you these last two weeks has involved stretching and relaxing one muscle or another. But what about the noggin?

You have been wracking your brain for hours and hours, day after day. Doesn't it need a healthy stretch, too? Or, perhaps a somewhat mindless pursuit to get you thinking about things other than numbers?

Growing evidence suggests that physical and mental exercise can alter specific brain regions, making far-reaching improvements in cognitive function, according to American Medical News, a publication of the American Medical Association.

"When you challenge the brain with new skills and new ways of doing things, it increases connections in the brain," says Ericka P. Simpson, MD, a neurologist who co-directs the MDA Neuromuscular Clinics. "It increases synaptic density."

Following are sites with fun puzzles and games that we have come across. Please let us know about your favorite sites.

• Brain Food – Logic puzzles, number puzzles, word boxes – this site has many!
• Brain Teaser Central – Brain-teasers, logic puzzles, and optical illusions abound on this site.
• 3D Logic – Link pairs of like-colored squares to complete a cube. Don't be fooled by how quickly you breeze through the first several levels. It will get awfully tough by Level 10.
• Brain Boosters – Challenge your brain in various categories, such as lateral thinking, reasoning and spatial awareness.
• Antaris – Maybe you just want to blow up stuff. If that's the case, grab the guns of the Antaris and protect your fleet from asteroids and enemy spaceships.

See the complete library of Workplace Fitness exercises.