RAT: You can modify the sign for mouse by using an "R" and it will mean "RAT."
There is another version of "mouse" that I don't see too often. In this
version you use your "pinkie." Hold your
extended pinkie off to one side of your nose. Move your hand sideways brushing
your pinkie past
your nose. Use a double movement.
Note: you don't actually have to "touch" the nose.
MOUSE: Version 2
Some people say that you should use the pinkie finger to mean "mouse" and
the index finger to mean "rat." I don't agree with that, but
I thought I'd mention it just so you'll be aware of it. I
recommend you sign "mouse" with the index finger and "rat" with an
"R"-hand and only use the "pinkie" version above as a version of the
sign for "immature."