I got my wife a Chia Pet for Christmas last year. She duly daubed the clay with seeds and I've been keeping it watered. It never really became bushy with plant-life, but it's still got growths, including one stem that spans about two feet. Given all I've been adding is water, and my understanding is that plants get their nutrients from the soil (or the water they're in for water plants) rather than from sunlight, where do Chia Pets pull it in? Is it just eking out on the coating on the initial seeding? Pulling it from the clay? Surviving on organic detritus in the tap water or which has settled on the clay?
Most of the bulk of a growing plant comes from carbon dioxide in the air, and water, not from the soil.
All plant seeds are basically concentrated packages of everything except CO2 and water that the plant needs to start growing.
"Chia pets" are not designed to grow the seeds into mature plants. When planted in normal soil, chia is a herb about 5 or 6 feet tall, which is grown commercially for its edible seed.