The floating test

A class wants to find out how the kind of water affects whether a small object floats. They prepare two clear cups with the same amount of water. Cup A has plain water. They add one teaspoon of salt to Cup B and stir until it dissolves. They place the same object gently into each cup and observe. To make a fair test, they keep the cups, water amounts and object the same. The one thing they change is whether the water has salt. They repeat the test to check that the result is reliable.