Proper & COmmon Nouns

[yas61]

Proper nouns sometimes becomes common nouns and vice versa. Is this statement true or false???? Can I get few examples????

[Tdol]

Xerox started life as a proper noun, but became a common noun referring to any photocopy.

[Casiopea]

Inventions are named after their inventors;e.g., A guy named John invented "the john", USA slang for toilet.