Although students learn this topic in Algebra 1, I always spend a day going over it again. I use this just prior to teaching Section 9.4 of the Prentice Hall CA Algebra 2 textbook.
The lesson shows how to factor both the numerator and denominator in order to find common factors. It reminds them to never cancel parts -- to cancel factors.
This lesson is needed in order to proceed with Common Core Standard A-REI 7
Understand that rational expressions form a system analogous to the rational numbers, closed under addition, subtraction, multiplication, and division by a nonzero rational expression; add, subtract, multiply, and divide rational expressions.