I had a white onion that I never used and for some reason instead of rotting, it sprouted. I put it in a cup of water on the window sill and now it's got a nice, six inch, firm green sprout on the top and little roots are coming out of the bottom. Can I plant this? What will it do if I plant it? Will it make more onions or what? I have never had any type of garden or anything and I know absolutely nothing about how nature works. But some friends and I were talking about starting a garden and it would be fun to plant my little sprout if I can.