I sort of fell into making one of these as a kid. I had a left a jar of water on the window sill in my bedroom absentmindedly. I noticed later that a wasp had fallen into it and drowned. Then I noticed it was starting to get fuzzy. Then a couple flies also fell into it. I thought it was interesting, so I threw a bit of algae from a fish tank in and sealed the jar closed. It grew into a big green blob and stayed green and "healthy" for at least twenty years, certainly well after I'd left home. But one day I went back to my parent's house and it was gone.