The first step is is to find a frame of capped brood and push a pvc pipe deep into the brood. There are exactly 263 cells that fit in the area enclosed by the pipe, so we count how many of these are empty and subtract that number from 263 to get the sample size for each hive. Next we pour a small amount (about 4 ounces) of liquid nitrogen into the pipe to create a freeze seal; ie this freezes the pipe to the brood cells it is in contact with. Next we pour about 8 ounces into the pipe and wait for it to evaporate off. This is the part that actually freeze kills the brood.