this is pi
but how do we calculate it?
first we'll draw a perfect circle
and then it's diameter
if we take the diameter to equal one unit
then measuring the circumference
will give us pi
in school we learn that pi is equal to the circumference over the diameter
so we'll take a look at the circle again
and we have a diameter of one
and the measuring the circumference
is
three point one four one five nine and
so on
which is equal to pi
now we can take any size circle 
and measure their circumference diameter ratio
and we see that we always get 
the constant pi
regardless of size
pi is going to be the constant of any
circle that exists
