Infants enter the world wailing. Research shows they begin laughing as soon as 17 days after that. Tears and laughter are basic human instincts, but, as we become adults and experience ever more of the world's travails, our smiles and laughter come far less frequently.
初生嬰兒以大哭降臨新世界，研究顯示，最快 17 天後他們就會發出咯咯咯的笑聲，可見哭與笑都是人類與生俱來的本能。