The grey hairs in a man's beard to the black ones.
'If a man hide himself so that his wife could not see him, and he wanted her to know where he was, yet had promised not to speak, laugh, cry, sneeze, cough, nor move his hands nor feet, how could he do so ?' ANS.

Whistle.
'The strongest man cannot stand against me.

I can knock him down, yet I do not hurt him.

He feels better for my having knocked him down.

What am I ?' ANS.