Case Closed(1994)

Meitantei Conan

Precocious high school student Jimmy Kudo uses his keen powers of observation and astute intuition to solve mysteries that have left law enforcement officials baffled. Hot on the trail of a suspect, Jimmy is accosted from behind and fed a strange chemical which physically transforms him into a grade schooler! Taking on the pseudonym Conan Edogawa, he attempts to track down the people who did this to him. But until he finds a cure for his bizarre condition, Jimmy continues to help the police solve their toughest cases. Can you crack the case before Conan does?

(Source: VIZ Media)

Note: Won the 46th Shogakukan Manga Award in the shounen category in 2001.