6x31 - Dragon Ball Z: Dragon Box

By using the merging technique of Fusion, which Goku learned from the Metamorian race, it might be possible to resist Boo! The Z Warriors may have been grasping at straws, but the Fusion technique would only work with those of very similar body sizes and strengths. As such, Goku began teaching the process of Fusion to Goten and Trunks, who both had similar builds and unmeasured power!