I learned it as:
A woodchuck would chuck all the wood he could chuck
If a woodchuck could chuck wood.
However, science marches on. For the definitive answer, see The Straight Dope archives:
http://www.straightdope.com/classics/a2_083a.htmlDear Cecil:
With regard to how much wood a woodchuck could chuck, etc., I am happy to say that science marches on, and the quaint but oh-so- unscientific answer you gave has been replaced with a modern one. See the enclosed article in the Wall Street Journal. --Randy B., Los Angeles
Dear Randy:
The article reports on the work of New York state wildlife expert Richard Thomas, who found that a woodchuck could (and does) chuck around 35 cubic feet of dirt in the course of digging a burrow. Thomas reasoned that if a woodchuck could chuck wood, he would chuck an amount equivalent to the weight of the dirt, or 700 pounds. We are pleased to know this, of course. But it sure isn't easy to fit into a snappy verse.
--CECIL ADAMS