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.
Pens and pencils and their parts - synonyms or related words. ballpoint. noun. a pen with a very small ball at the end from which ink flows as you write. Biro. British a plastic pen that uses a small metal ball at its point to move the ink inside the pen onto paper. cartridge. noun.