Emer,
this activity naturally lends itself to a discussion of fair testing. The children can predict if one liquid will more of an effect than another, based on their experience and understanding. If they find another liquid has a greater effect, they might discuss if the test was ‘fair’. Were all the egg shells the same thickness at the start? Were the jars kept in the same conditions, was the same amount of liquid added etc.
The next step arises perfectly from the activity.