I got the idea for this experiment from this blog. We just modified it slightly.
The objective is to show how our stomach acids aid the digestion process. There is a bit of gross factor here, so don't read on if you are sensitive :)
A zip lock bag
Some white vinager
A cracker (or cheerios or a piece of bread, etc)
Premoist the cracker. Explain to the child that this is what salive and mastication do for us.
Fill the bag with vinager. Explain that our stomachs hold acid inside, and briefly describe its properties.
Drop the food in. Close the bag tightly and swish the contents around, gently massaging the bag, too, to emulate the action of the stomach walls.
Describe what happens. Make notes and observations.
(My boys loved it!!!) :)