Take a 500 mL flask, insert a one-hole stopper with an 8 inch glass tube, attach two test tube clamps to either side for effect, and you have an artificial firefly. To demonstrate the presence and effect of air pressure, all you have to do is light a candle or bunsen burner, heat the bottom of the flask for a minute, and then invert the glass tube in a cup full of water with red food coloring. As the air inside the flask cools, the pressure will become lower than the pressure outside the flask, and water will get pushed up into the firefly. It creates the illusion that the firefly is sucking blood from the cup.
LL Demos & Labs: Wow, That’s a Big Sucker!