Other sourcE:

Tienshinhan, Krillin, Yamcha, and Chaozu all are heading toward’s Korin’s Tower, and are training on the way. Mr. Popo finally gives Goku an opponent to train with, and it turns out to be a clay version of Goku. Their skills are exactly the same, the only difference is that the clay Goku has no interference in his mind, and can concentrate completely on fighting. This shows Goku that he needs to quiet his mind, and he begins to realize how to achieve that.