What is causing white smoke to come out of tail pipes all of a sudden?

What is your question?

The temp was fine, no warning lights came on. I pulled over and it kept smoking. Drive it home, it smoked intermittently. it sat for a few days. A mechanic looked at it, and couldn't get it to smoke again. I drove it to work today, made it about 15 minutes into the drive, and it started smoking really bad again. White smoke. What could it be?