Brain

To sign brain, form your dominant hand into a claw and and touch the tips of your fingers and thumb to one side of your head (that is, one side of your parietal lobe, if you want to be more exact).

Brain is signed just a bit like lettuce, so be careful not to get the two confused, as lettuce is signed by taking a looser claw hand and having only the base of your palm touch one side of your forehead.