leaven
noun /ˈlevn/
/ˈlevn/
[uncountable]- a substance, especially yeast, that is added to bread before it is cooked to make it rise
发酵剂；（尤指）酵母，面肥 - (figurative) A few jokes add leaven to a boring speech.
几句笑话可给枯燥无味的演讲增添活跃的气氛。
词源Middle English: from Old French levain, based on Latin levamen ‘relief’ (literally ‘means of raising’), from levare ‘to lift’. - (figurative) A few jokes add leaven to a boring speech.