Since when, daviet, have you found agar plates being sold in your local super market?
If you have the right matterials then you can do Louis Pasteur's experiment.
This involves having two glasses, one of which you keep open as the control, the other you attach a swan neck fitting (it looks like an S on its side).
In both glasses (which should be clean) pour some meat puree (its like liquidified meat... bleagh) and leave for a day or two.
(Note: the swan neck should not have touched or come in contact with the meat)
You will notice that the glass without the swan neck is bad smelling and has bacteria growing inside of it. The glass with the swan neck however has no bacteria and doesn't smell (as bad).
This experiment shows that microbes cause disease and the swan neck is to show that they are in the air and can get into the swan neck, but are too heavy to get back out or into the meat.