Library paste, so tasty I once in kindergarten wrapped some up in a paper towel and took it home for later, is mainly flour and water, with some sort of minty additive. I can taste it now. Also contains alum, which is toxic in high doses, but who knew that.
The question is not why do kids eat it, but why do they stop?
library paste link, including a photograph of let this be a warning to you (scroll down).