That joke starts with this

As we all know, it takes 1 calorie to heat 1 gram of water 1 degree
centigrade. Translated into meaningful terms, this means that if you eat a
very cold dessert (generally consisting of water in large part), the
natural
processes which raise the consumed dessert to body temperature during the
digestive cycle literally sucks the calories out of the only available
source, your body fat.

For example, a dessert served and eaten at near 0 degrees C (32.2 degrees
F)
will, in a short time, be raised to the normal body temperature of 37
degrees C (98.6 degrees F). For each gram of dessert eaten, that process
takes approximately 37 calories as stated above. The average dessert
portion
is 6 oz, or 168 grams. Therefore, by operation of thermodynamic law, 6,216
calories (1 cal./gm/deg. x 37 deg. x 168 g) are extracted from body fat as
the dessert's temperature is normalized.


Then continues with drinking cold beer in a frosted glass, something about pizza, more beer followed by icecream...