Lawrence Hall of Science
The Math Page: Skill in Algebra: Reciprocals and Zero
Here is a site that clearly and thoroughly explains many topics related to reciprocals and the number zero, including the zero product property and the notion that dividing by zero is undefinable. There are example problems solved,...
National Council of Teachers of Mathematics
The Math Forum: Ask Dr. Math: Zero as Denominator
Math Forum response which explains why zero can not be the denominator of any rational number.