To do the sign for "rat," brush your nose twice with an "R" hand.
In context it is possible to do this sign with just an index finger
instead of the "R." But lacking context, the "R" hand makes it
clear that you mean "rat" and not mouse.
The sign for "rat" is thus an initialized version of the sign for "mouse."
You will see the sign
MOUSE done with either an index finger or a
pinkie, but I recommend you stick with the index finger for mouse and the
"R" hand for RAT. (Unless your local Deaf or ASL instructor has some other
Note: The sign RAT is also sometimes used to mean "freshman." This is likely due to the Gallaudet University tradition of giving a rat to incoming freshmen.
You can learn American
Language (ASL) online at American Sign Language University ™
by Lifeprint.com © Dr. William Vicars