During the retreat, I got to practice mindfulness of the four elements, as explained in the Four Foundations of Mindfulness Sutta:
[One] contemplates this very body — however it stands, however it is disposed — in terms of properties: 'In this body there is the earth [solid] property, the liquid property, the fire [hot and cold] property, and the wind property.'
Walking the Covered Bridge trail near Cloud Mountain, I noticed the earth against my feet, and I saw the tall trees, and the big rocks, standing still, weighed down with seemingly complete solidity. There was also the gently cool breeze taking turn with the hot sun against my cheeks, and the sight of leaves dancing with the wind. And in the morning, the dew from the blades of grass on the path, that made my feet wet, just a little.
The four elements. One can decide on just one, and keep one's awareness on that one only. Or, one can simply notice whichever is most prominent at any moment.
This morning, back to my daily exercise routine at the Y, I played again with the four elements. Sensing the solidity of the bike handles under my hands, and the pedals supporting my feet. Feeling the rising heat from body working hard, and once in a while the cool air from the nearby fan. Breath getting increasingly labored, forcing more and more air in and out. And, after a while, droplets of sweat, to remind me of the liquid nature of this body.
However your body stands, however it is disposed, it is always possible to practice the four elements. According to Leigh, this practice can help us break down the appearance of solidity in one's body, other bodies and other things. It is a tool for experiencing not-self. I also found it a very grounding practice.