“My hypothesis was that there might be 50% air in the worst offenders, not 87%,” he says.

Hargreaves calculated the volume of each filled bag by submerging them under water and then monitoring the displacement. He then vacuum-sealed the contents in another bag and compared the difference.

In fact, bags of crisps are not filled with atmospheric air but nitrogen, because the oxygen in air would cause the crisps to go soft. Nitrogen gives the product a longer shelf life, and a 1994 study suggested that it makes the crisps tastier.

You can see the process of Hargreaves’ work in the video below: