You can grow the tops of green onions in your windowsill, if you buy a bunch of green onions and you just use the tops, place the white bulb part of the onion in a small glass of water and put on a windowsill or sunny place. I took this picture 10 days ago on the 15th (sorry it's a little blurry) but you can see where I cut off the green part of the onion.
Fast forward to yesterday, all I did was change the water every day and look how much they grew!!! I cut the tops again and used them in last nights dinner.
How cool is this! The first few days nothing happened and then literally overnight they sprouted up, I don't know how many times I will be able to do this but I'll keep trying until they stop growing.