Prerequisite concepts
Differential Calculus
Difference / Change
Variables can be held constant
Understanding both difference (how far apart two values are at one time) and change (how far apart the value of a single parameter is at two different times) is necessary for understanding derivatives.
When taking a derivative of a multivariable function, it is possible to consider a simpler situation in which some of the variables are considered to be constant.
Instructor guide
http://physics.oregonstate.edu/portfolioswiki/swbq:mvsw:mvswpizza
Pizza and Cola
This SWBQ provides an easy introduction to the graphical representation of functions of several variables.
Representations used
2D Graph