The World's Most Powerful Computer is a minor alter ego of Calvin's. He appeared in a single story arc (7/6 through 7/7 1993) as an upside-down cardboard box with drawn-on features, which claimed to have the answer to any question it might be asked.
It only "answered" two questions; one from Hobbes and one from Calvin's mother. Hobbes asked why the computer wore little red sneakers, and his the following panel showed Calvin chasing Hobbes for asking that and knowing it was him. Calvin's mom asked if he cleaned his room or just spent most of the day playing with his box. When asked that, Calvin pretended that he had a system error and she needed to ask another question, but she didn't fall for it. After that, she sent Calvin up to his room to clean it.
The box for The World's Most Powerful Computer was smaller than the one used in most of Calvin's inventions. Calvin had drawn unfocused eyes and a squarish mouth onto the computer's "face", and on its side had written some nondescript text and drawn two buttons.