The sign for brain is done by tapping your forehead twice with the
tip of your index finger.
You can say that someone is very smart by calling him or her "big
brained" by holding an tight "C" hand near your forehead and then
quickly expanding it into a large, loose, C-hand.
BIG-BRAINED:

There is a sign that if interpreted literally would mean "double
brained." It is intended as a complement or a statement of
awe, admiration, or respect toward a person's genius or mental
prowess. What you do is you hold loose "S" hands
or semi-closed "C" hands in front of your head. One of the hands needs to
be in front of the other. It doesn't really matter which one is in
front, but I hold my dominant hand closer to my head and put my
non-dominant hand further out. Then you make a quick forward
movement with both hands, expanding them into fully open "loose" "C"
hands. As you do the movement it helps to move your mouth as if you
were saying "CHA" (which infers the idea of "very" large brained).
DOUBLE-BRAINED:

You can learn
American Sign Language (ASL) online at American Sign Language University ™
ASL resources by Lifeprint.com © Dr. William Vicars