In a very early episode of Dragon Ball, while trying to ride Kinton-un, Goku touched Chi-Chi's buttock by an accident. Brought up as a only daughter of Gyûmaô and living a secluded life, her face became red and beliving very wrongly that Goku liked her, she asks Goku that when she became older Goku will be the person to 'take' her. Simple Goku, thinking that she wanted him to have food or something, agreed.
Years later, Chi-Chi came back first hiding her name and as an opponent in Tenka'ichi Budôkai. Chi-Chi is angry that Goku had never came back to keep 'the promise' which Goku, of course, have no idea what that is about. Then she says that if Goku will beat her in the match, she will tell him what it is about. Goku does and is shocked to realize that little Chi-Chi had grown up as he had. Being a good natured man, Goku decide to keep his promise and announces that he will marry her infront of the whole crowd.
