An 18-time world champion retired because humans cannot beat computers at the game of Go. His name is Lee Se-Dol. Mr Lee is the only person in the world to beat Google's AlphaGo computer algorithm. This is an A.I. computer program to play Go. Mr Lee told reporters that: "Even if I become the number one, there is an entity that cannot be defeated. With the debut of A.I. in Go games, I've realized that I'm not at the top."

Go started in China 3,000 years ago. Today, 46 million people play it in 75 countries. It is the oldest board game in the world. Mr Lee started playing at the age of five. He turned professional when he was 12. He played five games against Google's AlphaGo and lost four of them. He said he won because of a "bug". The bug did not know what to do when Mr Lee made a "tricky" move. Today's Go programs are more powerful than the one that beat Mr Lee.