Well. This is a halfhearted attempt.
The Pan Is closed.(more appropriately, you have put a lid on the pan). Biologists claim that on increasing the temperature of the water suddenly the frog would jump out if it has the means to do so. However, the cruel guy who put the frog in the boiling water in the first place also put a lid on the pan. Thus, the frog jumped but couldn't get out of the pan and hence, died. Sad.
EDIT: Another half hearted attempt.
Who killed the frog? The frog itself. Because the water was too hot, it couldn't jump out quickly enough and died. (You could also plug in some morality lessons here.. But I digress)