The total amount of fat in the world stays constant over time. Fat can never be created or destroyed, only gained or lost. When one person gains weight, another loses it, and vice versa.
Julia seems to be shedding off the pounds whereas Lois is now sporting some thunder thighs- it must be because of the Conservation of Fat Theory.
When a dark haired individual inexplicably grows patches of red or blonde facial hair in between one's normal black or brown beard- so much so that the beard remains a secret to everyone but the beard wearer himself.
Henry's stubble is rather elusive to the eye. They say it's a secret beard.
When the tip of one's nose is so covered in blackheads that it resembles a potato.
Jack hasn't showered since last Thursday, it's no wonder he has a spud nose.