Tasks that are difficult to learn are the most effective, If they simply exercised, they didn’t retain the neurons. Learning must occur, and it must require some effort. So if you exercise, you will produce more neurons. If you do mental training you’ll keep alive more cells that you produced. And if you do both, now you have the best of both worlds: you’re making more cells and keeping more alive. The effort part is key. We need to learn things that are new, and we need to keep it challenging.”