Grow your own onions

I've seen many posts online lately from people saying that they were able to grow green onions from used onions. Oh really....? I gave it a shot. I first chopped up some of the onion down to about half an inch from the bulb. I then put the bulb in a small glass of water. And wouldn't you know it - the onion started growing back! So get twice the amount of onions for the price of one!