Meet the Bombardier Beetle. Cute little bugger, isn't he?
Like your average frat boy, it is capable of combining volatile chemicals in its own body and then expelling those noxious chemicals out of its ass. In the beetles' case, those chemicals are Hydroquinone and Hydrogen Peroxide, It stores them in separate compartments in its lower abdomen. When threatened, it triggers muscles that mix and expel the two chemicals; this usually results in a loud "POP!"
(Want to see one in action? A beetle, not a frat boy. Here: http://www.youtube.com/watch?v=Pib9qT-pccI )
Why the interest? Because my toddler, Wee Ginge, is doing her very best to impersonate a Bombardier Beetle today. No one can keep a straight face, as she keeps startling herself with large explosions.