Why is the United States often referred to as a "melting pot"?

Various cultures from many countries have "melted" together to create what America is and how American culture is defined today.
Most American citizens' native customs and traditions will "melt" away once they move to the United States and start experiencing American culture.
Most states in the U.S. have higher average temperatures than other countries. The high heat index has earned America the nickname, the "melting pot."
Sometimes, people may feel anger toward immigrants from other countries, but society encourages people to accept others and "melt" those feelings of hostility.