Trim the green onion tops, leaving about 1 to 2 inches of white bulb and roots
Place the roots in a small glass or jar
Add enough water to cover only the roots and the bottom of the white part
Keep the green tops above the waterline
Put the jar in a bright windowsill or other spot with indirect sunlight
Change the water every 1 to 2 days
Refill with fresh water as needed to keep the roots covered
Harvest the green tops when they reach the desired length
Trim only the tops and leave the roots in the water for regrowth
Replace the onions when growth slows or they become weak