Through the process of inquiry this is a great way to introduce the science concepts of density and buoyancy to young students as well as the scientific method. Could be used for elementary/middle school students.
The lesson (anywhere from 30 min - 45 min depending on how you modify it for yourself) plan includes:
-Prior Knowledge that students are assumed to have
-Scientific explanations involved in the activity
The tower I usually build is similar to the one Steve Spangler has used. You can always use a variety of other liquids and objects.
In this activity students will through a series of questions conducted by the teacher:
-Apply an inquiry process to determine what causes objects/liquids to float in relation to one another.
-Develop an understanding of density; mass per unit volume.
-Develop an understanding of buoyancy; objects less dense than the surrounding fluid will float to the top of that fluid/objects more dense than the surrounding fluid will float to the bottom of that fluid.
-Determine where objects will fall among several layers of different liquids.