Six cool facts about bones:

Today I'm going to share six facts about one of my favorite parts of the human body: BONES!

• Bones have the tensile strength of cast iron but at 1/3 the weight!
• Tensile strength is defined as "the resistance of a material to breaking under tension". This means that it takes the same amount of force to break your bones as a piece of cast iron the same size and shape. Tensile strength references forces that elongate a material, not compress it: For example when you hold both ends of a stick and bend them together to snap the stick in half. This isn't the same as compressive force. 
• Your bones are composed of roughly 15% water, 30% collagen, and 55% crystallized mineral salts. 
• The most abundant crystallized mineral salt being Calcium Phosphate. (Which is why people tell you to get your calcium to keep your bones strong.)
• In fact, our bones store 99% of all calcium reserves in the body
• The rigidity in your bones comes from the salt crystals and minerals while the strength comes from collagen.
• Adults have 206 bones in their body. 
• When you are a baby you have more than this (the exact number differs depending on which source you consult), but suffice it to say as a baby you have much more cartilage and not all of your bones are fused so that you can pass through the birth canal.
• 126 of our bones are found in our limbs and the other 80 of them are found in the skull, spine, and ribs.
• We have two types of bone structure: spongy and compact.
• Compact bone comprises long bones, as well as a hard protective coating on irregular, flat, short, and sesamoid bones. It is stronger than spongy bone, and compact bone in long bones contains red marrow which makes our blood!
• Spongy bone makes up the interior of the ends of long bones as well as the interiors of irregular, flat, short, and sesamoid bones. Not surprisingly, it looks like a sponge! Spongy bone contains mostly yellow bone marrow, which stores fats to be used for energy.
• You have only red marrow when you are born. As you age it changes to primarily yellow marrow. If you end up losing a lot of blood, becoming exposed to radiation, suffering from anemia, etc. the yellow marrow will convert back to red marrow until your blood supply is stable again.  
• Bones are governed by Wolff's Law, which states that bones respond to forces and stresses placed on them.
• This means that if you sit on the couch all the time, your bones don't every experience any stress, and therefore will not be as dense or as strong as they could be. Conversely, if you practice weight lifting regularly, your bones will become stronger and denser due to the added weight and extra force being applied to them. Your body recognizes that your bones have been stressed and it will lay down more collagen in the areas that experienced the extra force.
• Bones are the only tissue in the entire body that will heal to be as stronger or stronger than it was before injury.

Go forth and share your newfound bone knowledge with the world... Because Science! (And lift something heavy to get that bone density while you're at it.) ^.~ 

Cheers!

-Fliff



*DISCLAIMER*
I'm not a doctor, as such I cannot actually recommend you go lift weight. Please check with your general doctor or health care professional before starting any exercise routine. Further, if you decide to get a personal trainer, be sure they're qualified and that you fully disclose any previous injuries or any medical conditions. Trainers ask because they need to know to make sure they are keeping you safe and helping prevent any injuries. :D