The design of a computer-supported serious game concerning inhibition skills in children with Attention Deficit/Hyperactivity Disorder (ADHD) is reported. The game consists of a series of activities, each eliciting the tendency to respond in an immediate, inadequate way. The game is based on the Dual Pathway Model of ADHD proposed by Sonuga-Barke. In the game, children must block impulsive tendencies, reflect upon the situation, inhibit irrelevant thoughts, and find the non-intuitive solution. In the game, the player personifies a superhero, who is asked to save a realm on the opposite side of the Earth (Antonyms) where things happen according to the opposite of the usual rules. The hero faces a series of challenges, in the form of mini-games, to free the planet from enemies crossing different scenarios. To succeed in the game, the player should change his/her attitude by thinking before performing any action rather than acting on impulse. The player is induced to be reflective and thoughtful as well. Results from the evaluation of a preliminary version of the serious game are reported. They support the notion that Antonyms is an adequate tool to lead children to inhibit their tendency to behave impulsively.