Instructables
Here's a brightly colored science experiment that not only looks cool, but allows students to develop their own understanding of density!    I used this experiment for a freshman Physical Science class, but it could be adapted for many ages and situations.  It's also an incredibly cheap experiment that can be done with virtually no lab equipment if necessary.


Objectives:

1. Explain density using examples.
2. Identify and use laboratory equipment.


Other potential objectives:
1. Measure mass.
2. Define mass.
3. Measure volume. 
4. Define volume.
5. Calculate density.
6. Explain density conceptually.
7. Compare the densities of different materials.
8. Design an experiment to test a hypothesis.


 
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Step 1: Materials

Picture of Materials
You can make this experiment anything you like.  I used it very early in the year.  Students had measured and timed things for the velocity unit, but I was still teaching measurement and introducing them to the lab.  I usually included every skill they could possibly practice.

Required:
Pipets (Can be purchased online from all kinds of vendors. Example: http://www.flinnsci.com/store/Scripts/prodView.asp?idproduct=14269 About $25 for 400)

You could substitute syringes, turkey basters, or some other device that would allow you to add the liquid to the bottom of a tube but pipets are so useful you might as well buy some if you don't have them.

Useful (but could substitute):

250mL beakers (4-6 of these)
Food coloring
Salt (about 36g = 6 Tablespoons)
Stirring rod
Measuring spoons
Test tube


Substitutions:
Cups or bowls instead of beakers
Natural dyes instead of food coloring (Think beets for pink etc.)
Sugar instead of salt (This is a little messier and if not well cleaned up more likely to be a problem but works just as well.)
Spoons instead of stirring rods.
Any kind of spoon that you use could be substituted for measuring spoons. You just need to know that you're adding a relatively standard amount more for each additional color.


Optional:
Electronic Scale or Triple Beam Balance for extension
Test tube rack
Test tube drying rack
Test tube brush (for cleaning)

These optional ones are more about teaching them about these pieces of laboratory equipment.