Volunteers were told to think about yawning while they breathed either normal air, pure oxygen, or an air mixture with an above-normal level of carbon dioxide.

Volunteers were told to think about yawning, while they breathed either normal air, pure oxygen, or an air mixture with an above-normal level of carbon dioxide.

Hydrogen is added to the air mixture, causing it to burn easily and powerfully while the oxygen increases the octane value of the fuel.