Man holes and manhole covers are round. The outside diameter of the cover is greater than the inside diameter of the hole, therefore the cover cannot fall into the hole regardless of how the cover is positioned relative to the hole. Some of the covers weigh upwards of 300 pounds.
I'm sorry I don't feel like taking this question seriously because I am SUPER bored right now, so instead I have decided to tell you how gummy bears were invented, which is much more delicious then manhole covers.
The gummy bear originated in Germany, where it is popular under the name Gummibär (help·info) (rubber bear) or Gummibärchen (help·info) (little rubber bear). Hans Riegel Sr., a candy maker from Bonn, started the Haribo company in 1920. In 1922, he invented the Dancing Bear, a fruit-flavored gum made in the shape of a bear. The success of the Dancing Bear's successor would later become Haribo's world-famous Gold-Bears candy product in 1967. Trolli is another gummi candy manufacturer and the first to introduce gummy worms in 1981. Gummi bears are also one of the few types of candy to have inspired a television show.
I copy and pasted that because I am a lazy American