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.
A reed pen is cut from a reed or bamboo, with a slit in a narrow tip. Its mechanism is essentially similar to that of a quill. The reed pen has almost disappeared but it is still used by young school students in some parts of India and Pakistan, who learn to write with them on small timber boards known as Takhti.