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.
The name for a male swan is a cob. I believe that the pen knife was used to sharpen quills as you have explained, but the writing implement was called a quill, not a pen. The name penknife came to be because the best quills for writing came from a young male swan and young male swans are called pens..