Makoto is surprised by Saki's confession of love and has to clarify that he is not the girl Saki thinks he is. However, Saki is undeterred and throws herself wholeheartedly into the mission of becoming Makoto's first love. 
How can you be so sure?
Wieso bist du so überzeugt davon?
¿Y cómo estás tan seguro?
¿Cómo estás tan seguro?
Qu’est-ce que t’en sais ?
Come puoi dirlo con certezza?
E como pode ter essa certeza?
Тебе-то откуда знать?