Step 1: Getting Associated With the Circle.
r stands for radius, and radius stands for Really important.
Remember, radius really really is relevant.
Have you ever heard of diameter? Who needs diameter? Diameter is for fools.
Well you could use diameter, but then I won't be able to give you the neato way to remember the circumference equation in the next step (by the way, diameter is just radius multiplied by 2.)
To find the radius, measure from one point on the circle to the opposite side from that point using a ruler, that length is your diameter, divide that by two and BAM you've got yourself a pretty little radius.
Step 2: Circumference Equation.
Take your pick, you will get the same answer with each equation (if you don't, well then maybe you should just try coloring inside of the circles instead of trying to find their circumference.)
Circumference= Two x Pi x r
Circumference is equal to two multiplied by Pi multiplied by the radius of the circle.
Now, do you have the memory of a goldfish (I think their memory is only 3 seconds. Or maybe it was 2, or maybe it was 4? I don't remember). Here is an easy way to remember the circumference equation:
TWOPIES ARE BETTER THAN ONE
2 Pi r
Step 3: Application.
Here is a circle with a radius of 20 inches (not drawn to scale).
So lets plug in 20 as the value of r in C=2Pir
Circumference= 2Pi multiplied by 20
(You can use the Pi button on a calculator, or use the decimal form of Pi, which is 3.141592653589793238462643383279502884197169399375105820974944592307816406286208998628034825342117067982148086513282306647093844609550582231725359408128481117450284102701938521105559644622948954930381964428810975665933446128475648233786783165271201909145648566923460348610454326648213393607260249141273724587006606315588174881520920962829254091715364367892590360011330530548820466521384146951941511609.... and so on, but I would suggest using a rounded decimal of 3.14, which is commonly accepted as the decimal of Pi.)
So, here we go, using the pi button on a calculator:
And using the rounded decimal value of pi
C=2 x 3.14 x 20=125.6inches
And these answers are essentially the same, unless you are an architect of a cylindrical building, then those extra decimals may be of more importance, but for our puposes, lets just round and say the circumference of a circle with a radius of 20 inches is 125.6 inches.
Step 4: Now,
Want to find the circumference of a pizza, (or should i say pizza pie? Get it, pie, like pi, ha, punny) ?
Grab your self a slice of pizza, and measure with a ruler the length of the pizza slice from tip to crust.
Multiply that length by 2Pi, and voila (as the French would say) you can now pick up that hot chic at the pizza parlor by telling her "Hey, I just got a pizza with a circumference of 37.68 inches, would you like to come share it with me?"
And if she says no to that winner of a pick up line, then she just must be allergic to pizza.
(Note: this pick up line may be used on males as well, everyone knows that male+pizza=happy, but thats just a whole other equation that we won't worry about right now).