Every designer-experienced or not-aspires to live Aha! moments, but how to favor these moments, where original ideas emerge, is not always obvious. While design methods often focus on the concept space, this article highlights the role of knowledge structuration in idea generation. By analyzing two cases of idea generation sessions with C-K theory, we show that generativity relies on the designers' ability to reorganize their own knowledge basis and thus create new design rules. " Incompatible knowledge " especially helps fostering such restructuration, and therefore, provoking inspiration.