Re-Growing Celery, I mean how cool is that!
So all you have to do is;
Get yourself a celery clump and cut off the end (where the roots grow)
Then firmly poke 3 tooth picks around the base, so that it doesn't fall into the water completely,
Put it into a jar full of water, so that the root part is submerged in water.
Then wait 3-4 days and see if there is any movement or change.
Mine started to form a head.