2% of total bodyweight

Thrive on learning and mental challenges. The point of having a brain is precisely to learn and to adapt to challenging new environments. Once new neurons appear in your brain, where they stay in your brain and how long they survive depends on how you use them. "Use It or Lose It" does not mean "do crossword puzzle number 1,234,567". It means, "challenge your brain often with fundamentally new activities".

---
From SharpBrains.

This is the 5th out of 10 habits of highly effective brains, a brief, crappily titled but excellent article that you should read.