FIrst, we observed the powder, and , considering the context (grocery store) hypothesized about what it could be.
powdered milk, flour, rice flour, biscuit mix, pancake mix.
the kids tasted the stuff, and said that it tasted like flour. They still wanted to add baking soda to see if it would fizz, but I advised against it.
then we performed the cooking test. I mixed a small amount with water, then cooked it like a pancake. the powder definatly fluffed up during cooking, so it probably contains some baking soda. It also tasted great, so it is some sort of baking mix.
I then mixed it up as if it were biscuits, cooked it, and they tasted great.
I am secretly relieved that it wasn't plaster of paris or something. Tasting mysterious white powder is not always a great idea, but here it worked. FYI: when we invite people to come and eat at our house, we don't serve mystery food.