| 4 | 1921 | - 1921: Warren Harding president of the USA 1921. Warren Harding dies of an embolism in San Francisco. He had taken ill on 31 Jul 1921.
- 1921: Calvin Coolidge president of the USA 1921-1929. Calvin Coolidge, vice president under Warren Harding, sworn in as president the day after Harding dies.
- 1921: Artificial life begins -- the first robot built.
- 1921: John Larson, a University of California medical student, invented the modern lie detector (polygraph), used by police since 1924.
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1917 - 1921 (3 years)