Analysis of air bubbles trapped in fossils, such as in fossil amber, show that our air in earliest times contained about 35% oxygen. Today, the average oxygen content of our air is approximately 20%. In some larger, more polluted cities, oxygen content in the air has been measured at 12 - 15%.
There are 297 words which start with 'pen'. 3 letter words starting with 'pen': pen. 4 letter words starting with 'pen': pend. pens.
Look at the keyboard of any standard typewriter or computer. Q,W,E,R,T and Y are the first six letters. The first practical typewriter was patented in the United States in 1868 by Christopher Latham Sholes. His machine was known as the type-writer.