The human brain starts working the moment you are born and never stops until you stand up to speak in public.

George Jessel (1898-1981) American comedian, singer, songwriter, producer

Quoted in The Observer (7 Aug 1949)

Jessel was a frequent master of ceremonies, and often used the line. Variant: "The human brain is a wonderful thing. It starts working the moment you are born, and never stops until you stand up to speak in public."