In the familiar proverb, it's when "the mice will play"
Yes it's really useful, and fun :D. [DOC] [TLE] 12 Proverbs in Literature - De Gruyter [PAR] the mice will play; Birds of a feather / flock together ; Different strokes / for ... [then] 
we die]; Homo proponit, / Deus disponit  with its rhyming derivatives or coun- ...