At the right, that's wheat grass growing on a window sill, as shown below:
Wheat grass contains few calories but it's rich in vitamins, minerals, proteins and enzymes, and may improve a number of health conditions. You can read a lot about it at MedicalNewsToday.Com.
Other Green Drink recipes are available from Prevention.Com.
Outside-grown wheat grass is more nutritious than what is grown inside. Other kinds of grass can be used, too, such as barley. Grass growing in agricultural fields or gardens is fine, except be sure that it hasn't been poisoned with pesticides. Many Green Drink recipes don't include any grass at all.
At the left you see a tray in which barley seeds lie unburied atop the soil. A loose, moist soil not containing weed seeds was placed into the container, then atop the soil a thin layer of seeds was sprinkled. You can see that little or no soil shows between the seeds, and that roots emerge from the seeds.
At first the tray should be covered to prevent the soil and seeds from drying out. Before the seeds sprout it doesn't matter whether they are exposed to light or not.
Once the seeds sprout, however, they need all the sunlight they can get. At least until the seeds are sprouted and roots have penetrated the soil, the tray should be covered with something like a clear plastic bag that lets sunlight in, but doesn't let the moisture escape. A little ventilation may be needed to keep heat from accumulating and killing the plants.
At the right you can see how the blades are harvested, leaving plant bases to produce further blades.
It takes about 9 carrots to produce the 1½ cups of juice needed.
And then...
ENJOY!