Tomas was ten years old and quite sure of one thing: air has weight. His older sister Elisa laughed so hard that she nearly dropped her toast. "Air is nothing!" she said. "That's the whole point of it." Tomas said nothing. He took a flat football from the garage, put it on the kitchen scale and wrote down the number. Then he pumped the ball until it was hard as a stone and weighed it again.
The number was a few grams higher. Elisa picked up the ball, put it back on the scale and frowned. She checked the scale twice, once with a spoon on it, to make sure it was not broken. It was not. Their father, who had been listening from the doorway, explained that the same air presses on everything around us, even though we never feel it.
Tomas took the football to school and showed the class, and at the science fair he won a ribbon. Elisa, who had never admitted she was wrong in her life, became his assistant and gave herself the title "Head of the Air Department."