Last minute science project for a 3rd grader, any ideas??

You could show how cold water is heavier than warm water. All you really need for supplies is two 2-liter soda bottles (and that's kind of optional since other containers would work, water and food coloring. Fill one bottle 1/2 full with hot water dyed with red food color. Fill the other bottle with cold water dyed with blue food color. Poor the blue into the red and watch as the cold blue water displaces the hot red water and settles on the bottom...some water will mix and give you a purple layer in the middle but the overall idea will stay intact.

How about showing how yeast is alive? In this experiment you will grow yeast in different conditions to see which conditions will cause the yeast to be most active during fermentation. You will use balloons to trap the CO2 gasses released by the yeast during fermentation. Then you will submerge the balloons under water and use water displacement to measure the amount of gas in each balloon. 5 packets of dry yeast 5 recycled plastic liter bottles white table sugar salt baking soda vinegar water small balloons http://www.sciencebuddies.org/science-fair-projects/project_ideas/MicroBio_p011.shtml?from=TSW # string or clips for tying off balloons