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You Will Need
 A jar filled with spherical or oblate spheroid candies
 Calculator
 Tape measure
 Vernier calipers (optional)
 Computer with internet access (optional)
Steps

Step 1
Estimate jar capacity
Ask for or estimate the total volume of the jar as best you can. Convert the volume to milliliters.

Step 2
Determine whether the candies are spheres
Determine whether the candies are spheres. If they are balls, like gumballs or jawbreakers, they're spheres. If the candies are round, but longer than they are wide, they are "oblate spheroids."

Step 3
Find the volume of one candy
Find the volume of one candy, also in milliliters. First, find the radius of one candy, either my estimating, using a tape measure, or by using vernier calipers, which will provide the most precise measurement. If your candy is spherical, use the formula V = 4/3πr3, where r is the radius of one candy, in centimeters. Round pi to 3.142 if you don't have a scientific calculator.

Step 4
Determine percentage of volume used
Calculate the percentage of the total volume the candies take up in the jar. Calculate 64 percent of the jar's total volume if the candies are spheres, and calculate 66.5 percent of its volume if they are oblate spheroids.

Step 5
Figure it out
For spherical candies, divide your estimate for the size of one candy into 64 percent of the volume of the jar. For oblate spheroid candies, divide the average size of one candy into 66.5 percent of the volume. You've got the answer; now amaze your friends with your guess!