a) they required few, cheap materials (2 pop bottles and 1 connector). I found the set of connectors at Michaels for under $10,
b) it was fun for all ages (tested on 3 generations at B's parents' house),
and c) it allowed for creativity.
We put in blue food coloring, glitter, and a couple Monopoly houses. It was very Wizard of Oz. At one time, Polly Pocket also found herself caught inside a tornado. Which then resulted in my godson wanting to play with Polly Pocket all night, but that's another story.