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 earliest pens were made from hollow reeds and, later, hollow wing feathers of geese and swans, called quills, infused with ink. Steel pen nibs came into use in the early 1800s, and then fountain pens, which did not require a constant resupply of ink like previous incarnations, gained popularity.