Yeah I'm thinking the "mass in numbers" concept has something to do with it. Probably good for composting, but not much worms can do. Thanks for the info!

Look into "bokachi" with a search engine.

A 2-liter soda bottle is probably too small to really work well with worms.

10-gallon aquariums cost about $10 new (or a few bucks at yard sales), and are a practical size for a small demonstration bin. As long as you don't let too many castings accumulate, it will stay light enough to move around pretty easily (to bring to schools, etc.). Make a cover with a cardboard box or something to keep it dark inside and encourage the worms to be active right next to the glass.

5 gallon and 2.5 gallon aquariums are also commonly available, but actually cost a few dollars more than the more common 10 gallon. They're also probably too small for an easily sustainable bin.