In a medium bowl, stir together half of a bottle of lemon-lime soda and the grenadine. Pour it into your brain mold. Freeze for up to 6 hours, or overnight.
In a large punch bowl, add the remaining lemon-lime soda and the limeade concentrate, stirring until fully mixed. Add half of the sherbet in scoops and stir, letting sit for 30 minutes. Add the frozen brain, remaining sherbet and serve.