be another thing

To be a separate and very different situation, topic, idea, etc., that has a whole other set of difficulties or problems to consider or contend with. It's all well and good wanting to help your fellow man, but inviting strangers off the street to come live with us is another thing entirely! Oh gosh, the state of the roof is another thing—we don't have time to get into all that right now. Getting a bad grade is one thing. Hiding it and lying about it is another thing altogether and totally unacceptable.