Fill the glass will seltzer water until it is about 3/4 full. This experiment is a lot more fun with a taller glass.
Drop several raisins into the glass and observe. It might take a few seconds for something to start happening, so be sure to take a close look at how the bubbles of carbonation in the selzter are helping the raisins to float.
If you have them, try dropping in some other light, small objects, such as grains of rice or oats. How do they act? See if you can predict which objects will float and which will sink.