Calcium is the chemical element with
the symbol Ca and has an atomic number 20. It has an atomic
mass of 40.078 amu. 
Things you might not know about
Calcium:
- Calcium is the 5th most abundant element in
the Earth's crust, present at a level of about 3% in the oceans and
soil.
- The element name "calcium" comes from the
Latin word "calcis" meaning "lime".
- It is used in the making of cheese, where
calcium ions influence the activity of rennin in bringing about the
coagulation of milk.
See GC2XAY2 - Hydrogen, for an explanation
about this series of caches.