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.
writing implement, late 13c., from Old French pene quill pen; feather (12c.) and directly from Latin penna a feather, plume, in plural a wing, in Late Latin, a pen for writing, from Old Latin petna, pesna, from PIE *pet-na-, suffixed form of root *pet- to rush; to fly (see petition (n.)).