In the familiar proverb, it's when "the mice will play"
... Rhyme (
When the cat is away, the mice will play); Ellipsis (Once bitten, twice shy) ... 
Further typical features of the proverb are its shortness (average: seven words), 
and... [DOC] [TLE] Proverb familiarity and the mental status examination - ProQuest [PAR] Differences in patients' familiarity with proverbs may lead to incorrect assessment
.