Pennies to Heaven is a Fermi Problem, basically a “headmath” experiment. Fermi Problems, originally developed by Enrico Fermi, one of the greatest experimental and theoretical physicists of the 20^th century, are real-world estimation problems. So we ask, “If we had a stack of pennies as tall as the Empire State Building, how big a room would we need to hold them?” Like most Fermi problems the answer to this one is a delightful surprise and requires us to think out-of-the-box. Always ask, “What do you guess?” “Would you need a whole house or something bigger, just your bedroom, or a closet, or something even smaller?”