body’s usual sensors for low oxygen or high carbon dioxide levels in the blood but in fooling the diaphragm. When you breathe in, you’re contracting the muscle of your diaphragm, pulling it flat so that the volume of your chest increases – and air is drawn into your lungs. When you hold your breath, you keep your diaphragm in this contracted state. Artificially raising oxygen levels and reducing carbon dioxide levels before a breath-hold, as in the Birmingham radiotherapy experiments, may work by delaying fatigue in the diaphragm. And – not so useful if you’re trying to keep your chest perfectly still – breathing out a little air lets the diaphragm relax a little, and helps you to prolong a breath-hold, exactly as I found when attempting my breath-hold. And, so, it’s your diaphragm, the main muscle of breathing, that is also in charge when it comes to reaching the breakpoint of your breath-hold. Eventually, even if you’ve fooled it for a while, the signals from the diaphragm are just too strong and you have to give in – and take a breath.