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.
fountain pen-a pen that is supplied with ink from a reservoir in its barrel. nib, pen nib-the writing point of a pen. quill, quill pen-pen made from a bird's feather. Sharpie-a pen with indelible ink that will write on any surface. writing implement-an implement that is used to write.