
While slope is used to describe the measurement of steepness of a straight line, it can also be used to describe the crosscountry ski slopes that Oscar likes to frequent.
Below is a breakdown of the types of slopes that Oscar encounters on his crosscountry graphing adventures. 

Positive Slopes:

Positive Slope
Lines that slant "uphill" on the graph,
from left to right, have a positive slope.
Such "hills" require Oscar to increase his energy level to get up these hills.
He must add (+) energy
to get up the hill.
Graphed lines that go
"uphill", from left to right, have positive slope. 

Negative Slopes:

Negative Slope
Lines that slant "downhill" on the graph,
from left to right, have a negative slope.
Such "hills" require that Oscar decrease his energy level in an
attempt to slow down.
He must subtract () energy
to slow down.
Graphed lines that go
"downhill", from left to right, have negative slope. 

Zero Slopes:

Zero Slope
Lines that are horizontal
(straight across from left to right)
have zero slope.
Such "paths" do not go uphill or downhill. Oscar does not need to increase,
or decrease, his energy level.
The change in his energy level is zero.
Graphed lines that are horizontal have a
slope
of zero. 

Horizontal lines have lots of "run", but no "rise". Therefore, rise/run = 0/number = 0. 
No Slope Exists (or Undefined Slope):

No Slope Exists
or Undefined Slope
Lines that are vertical (straight up and down) have no slope (or undefined slope). This means the slope does not exist.
It does not mean the slope is zero.
Oscar cannot ski on such "paths".
Broken bones await him at the bottom.
Graphed lines that are vertical have no slope
(or undefined slope). 

Vertical lines have lots of "rise", but no "run". Therefore, rise/run = number/0 = undefined. 
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