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."