a) wreck your brain(s) - not a common expression. It would mean to destroy your brain, in my imagination perhaps through alcohol or drug abuse. b) rack your brain - to think hard looking for a solution to a problem. Brain is generally used in the singular. c) use your brain(s) - don't be stupid, think about it. Brain can be singular or plural. When it is plural it refers to your intelligence more generally rather than the organ of the brain.