I read about this tip and gave it a try the other day. With a large shop bought celery , I kept its stalk head with few lime green leaves. Put it in a glass with fresh water on the windowsill. After a few days the leaves become darker green and some roots start to form. I then cut the head into 4 parts, pot it up with seeding compost. They are now growing away as 4 individual celery plants!
Here's another tip for those who do grow celery. Don't lift the entire plant when harvesting but cut it off about an inch above ground. It will re-sprout and you'll get a second crop of smaller but perfectly usable stalks.