two things that dissolve in water

Can't help you with the Listerine, but the peanuts I think you mean, are made of potato starch.

I ordered something from some eco-friendly company one time that used those water-soluble packing peanuts, and the guy must have told me you could eat them, because I did. It was really stupid, in retrospect, because... who knows where those things have been and how many rats peed on them. But I was sort of enjoying eating them, even though they had no flavor to speak of. I won't be doing that again, because I wound up with one of the worse cases of the runs in recent memory.

My wife sometimes ordered from a beauty supply company that would use peanuts that you would dump into your bath water and it would dissolve into bath oil and bubbles. Pretty neat idea! check wiki.

http://en.wikipedia.org/wiki/Packing_peanuts

The last time I looked at the ingredient list 'Listerine strips' were using a material called 'Pullulan', a microbe generated polysaccharide biopolymer as the film former. But, perhaps the recipe has changed since then.

Our packing peanuts are made from cornstarch.

Yup, the ones I ate reminded me more of corn that potatoes.

One could say that you were corn holed and then got the runs.