The Truth of the Breath
Some people practice throughout their entire lives just by paying attention to their breathing. Everything that is true about anything is true about breath: it’s impermanent; it arises and it passes away. Yet if you didn’t breathe, you would become uncomfortable; so then you would take in a big inhalation and feel comfortable again. But if you hold onto the breath, it’s no longer comfortable, so you have to breathe out again.
From Body as Body