Feel like your memory is getting worse with every passing birthday? It happens to all of us as we age, but a growing body of medical evidence suggests that lifelong stimulation is the key to building brain cells, staving off memory loss and maybe even preventing Alzheimer’s disease. Research has found that doing interesting work (paid or volunteer), pursuing hobbies and engaging in an active social life can help. Try challenging yourself with music or language lessons, plan a trip with friends, or just hunker down with a good crossword puzzle this weekend. And while you’re at it, do it all with a smile. Studies show that a positive emotional state is also good for your brain.