Stomachache

Little Susie, a six year old, complained, “Mommy, I’ve got a stomachache.”
“That’s because your stomach is empty,” the mother replied. “You would feel better if you had something in it.”
That afternoon, Susie heard her father complaining that he had a severe headache all day.
Susie perked up, “Daddy, you have a headache because your head’s empty”, she said. “You’d feel better if you had something in it.”