Learn a second language. Researchers in England found that people who learn a second language early had denser grey matter than those who spoke only one language.
"The scans revealed the density of the grey matter in the left inferior parietal cortex of the brain was greater in bilinguals than in those without a second language.
The effect was particularly noticeable in the "early" bilinguals, the findings published in the journal Nature revealed."
The research also concludes that older learners will have more difficulty in learning a second language (that light brain again).
There are other reasons to learn a second (or a third) language. It gives you an insight on how other people think, what they feel, how they look at life. It's hard to hate someone once you understand them. Charlemagne once said: "To know another language is to have a second soul."
2005 has been designated as the Year of Languages in the US. Ironically, when you read the contents of the website, they're talking about people learning English as a language:
"Educators across the country recognize that there has never been a better time to focus public attention on the personal, professional and cultural benefits of gaining proficiency in English as well as other languages.
I'm not sure if this is an indication of the numbers of immigrants to the US who are not learning English, or of the decline of the Americans' proficiency in their own language.
Nervertheless, learning a new language is an exciting, challenging endeavour. I recommend it to anyone who wants to exercise their brain.