This is much more than a saying. When actually practiced, has many health benefits. Here a few helpful links:

To help me, I breathe in through my nose and pretend my stomach/chest is a balloon being filled up, then I slowly blow out through my nose.

I also got the book, A Boy and A Bear: A children’s relaxation book, to teach kids how to take deep breaths. It has worked with many kids. Both my kids take deep breaths on their own. When my 18 month old feels my tension, she’ start taking deep breaths. When my sixyear old feels my tension, he tells me to take deep breaths and the number of deep breaths he recommends depends on how upset he thinks I am. He has prescribed up to eight! 😉

The Relaxation & Stress Reduction Workbook is a great adult book for relaxation techniques.