I saw this around somewhere and had to try it. The idea is that you can put the white-ends of scallions in water and they will grow, and that you can just keep cutting them off and letting them regrow indefinitely. Last week sometime I took all the white ends of some scallions and put them in water. A few days later, I added some other scallions I'd used. Kept the water changed every few days, used some as needed. This is the result!
It seems like it takes about 4 days to grow from nubbins to a full, useable stalk. I'm considering getting another bunch of stems to use and make starts, because what I have here I use most or all of at once and then have to wait. I'd like to have at least a couple different batches going so that I always have some available to use.
*Tip: Cut them all the way down into the white part of the onion, past any spots where they split off. Otherwise they seem to grow slower and not as sturdily, the tips get brown, and you end up with some spots that just don't grow again.
Shown here with rocks in the bottom to support the roots that wanted to tip over due to being top-heavy. Won't be necessary with the bowl a little more full, but this almost makes it look like a real house plant!