Nintendo today announced that its brain training titles are selling madly worldwide. Dr Kawashima’s Brain Training: How Old Is Your Brain? already sold 3.5 million units, while Brain Age: Train Your Brain in Minutes a Day surpassed the 4 million mark. What's more interesting is that sales are not going down, but up. If you don't know what these titles are, press 'read more' for details and who knows, you could start training your brain today.