Funny story

Ruth realizes that birds, in zoos, are in cages with roofs.

 Otherwise, the birds could fly away.
 Isaac can't believe this is a new thought for Ruth.
 Dad's pretty amused also.
 Ruth has just realized that butterflies, too, have to be kept inside. Otherwise, butterflies would migrate.
 Seriously, Ruth?
 Well, I figured it out eventually.
 What's all this talk about birds? The cat wants in on the action.