As quiet as a mouse.
Mice are very quiet animals.
A person might be told to be quiet as a mouse if quiet is in order. Scientists
have determined that mice make ultrasonic sounds. These sounds are too high pitched
for the human ear. Mice rely heavily on scents and smells to communicate. They have very sensitive noses. They
also communicate by touch. This is why when people want other people to be very
quiet they say “be quiet as a mouse”.