Once there was a mediaeval torture device called a rack. It, to put it mildly, stretched and tore the limbs of its victims. So the word "rack" became an adjective for pain. Over time it evolved into how we use it today. It was found in print in 1680 as a figure of speech. ( source )  When we use it today we are saying that one has to think very hard or with strain. 

This phrase means something that requires little mental effort or intelligence to perform or understand. The term is often applied to decisions which are straightforward or sometimes to people who appear to lack intelligence. The origin of it isn't clear, it seems to have just come into being. Read about it here .

Strawberry Day World's Healthiest Foods: Strawberries Kahlua Day The Original Spirit of Veracruz, home to Kahlua The many flavors of Kahlua Kahlua Cocktails Kahlua Dessert Recipes Polar Bear Day National Geographic: Polar Bears Should the Polar Bear Still Sell Coca-Cola? No Brainer Day no-brain·er noun informal something that requires or involves little or no mental effort. synonyms: easy decision, easy task; foregone conclusion, sure thing, certainty, given Waaaay back in the day :) my friend and I had a little mantra, "Remain Insane, No Strain on the Brain."  That's kind of a tag line for no brainer day. Don't strain your brain today.  *wink*

It seems a little obvious to be grateful for the brain BUT have you ever really thought about it?  The brain that is.  Have you ever used your brain to think about the brain?  It is fascinating!  It controls our movements, our thoughts, our speech, even aspects of our personalities.  It is fascinating, delicate, intricate, and vital.  Due to a co-workers friend who has a brain injury we've been talking about the brain a lot at work recently.  And we are all fascinated all over again at the way the brain works and how it controls everything.  It's fascinating how it recovers from injury, what it does to protect itself and therefore the person, etc.  I'm so grateful, obviously, for the brain and I am in complete awe and fascinated all over again by its contributions.