Watch Willie Nelson, Sister Bobbie Rain Praise on the 'Laws of Nature'

Eco-conscious siblings collaborate on a new tune they joke is "like a science lesson"

Willie Nelson's self-penned "Laws of Nature" is a sweetly simplistic love letter to Mother Earth. The song, featured on the country icon's new December Day album of collaborations with his sister, Bobbie Nelson, extolls the virtues of the sun, rain, wind and air and reminds that they are life forces.

"I'm not really sure where I was coming from on that, or where I went. [Laughs] But it seemed to be the thing to write at the time," Willie says. "Somebody says it's like a science lesson."

"It is like a science lesson," Bobbie, a member of her brother's band since 1973, chimes in. "I learned a lot by listening to this song."

For the accompanying video, the Nelson siblings alternate performance clips from Willie's Pedernales Recordings Studio in Austin, Texas, with scenic shots of Texas farmland. Perhaps in an attempt to persuade the viewer to appreciate beauty all around, the landscape video mimics the view from a moving car. It's a fitting musical tribute from the Farm Aid co-founder, who also owns his own bio-diesel brand.

In fact, December Day was largely inspired by family jam sessions on board the Honeysuckle Rose, Willie's biodiesel-fueled tour bus. Released last week, the LP finds the Nelsons coupling new songs such as "Laws of Nature" with revamped hits including "Who'll Buy My Memories?" and covers of American classics, including Irving Berlin's "Alexander's Ragtime Band" and "What'll I Do." December Day is the first installment in Legacy Recordings' Willie's Stash series, a collection that will revisit decades of country hits by the American treasure, along with treasured new tunes.