Shaggy and Scooby-Doo: Get a Clue
Don't Get a Big Head
S2 E10:
Dr. Phineus Phibes is convinced that the sole reason he is continually defeated by Scooby and Shaggy is that he is simply not smart enough. Phibes uses a nano-formula that makes him super smart. It is up to Scooby and Shaggy to use "dumb gas" to make Dr. Phibes stupid again.