Now Hollis lets the air out of the ball and puts the flat ball back on the scale.

The display reads 580 grams again. The air left, and its 5 grams of mass left with it.

The pumped ball reads 585 grams; with the air let out, the same ball reads 580 grams.

Next, Hollis pumps up a flat soccer ball.

Before pumping, its mass is 410 grams. After pumping, its mass is 415 grams.

The soccer ball before and after pumping: 410 grams, then 415 grams.

The two balls, flat and pumped up, and the extra mass each time.