The primary unit is studying their digestive systems. They put food into their stomachs (baggies), added the digestive juices (vinegar), and then churned it around to see the result.
They took careful notes during the process, noting what the food looked like before and after digestion. One of the bags exploded during the process, providing an excellent impromptu example of vomiting!
Over dinner, be sure to ask your child to tell you about how their food is digesting!