This is how tongba is made. Tongba is made from the crop we know as millet.
But to be more precise and for your knowldege here's the info.
Plant Family / Families: Poaceae
Scientific Name: Eleusine coracana

Eleusine coracana is an annual plant native to Africa, where it has also been grown as a food crop since ancient times. It is similarly cultivated throughout Asia, where it is believed to have been introduced as a cereal crop thousands of years ago. The plant has easily adapted to higher elevations and is grown in the Himalayas; the field in this photograph was growing at an altitude around 2300m.
Due to its wide cultivation, Eleusine coracana goes by a variety of common names such as

ENGLISH : Indian millet, Ragi millet, Finger millet, African finger millet.
NEPALESE :  Kodo
JAPANESE : Shikoku bie
HINDI : Ragi
THAI : Khao pang samngam

Finger millet, one of the English common names, tolerates poor soils and low rainfall. It also lasts a very long time in storage. These features, combined with its very high nutritional value (higher in protein, fats and minerals than corn or rice), makes it an essential crop to some of the poorest farmers in the world.
As a member of the grass family, the plant has strap-like green leaves with parallel venation. The seed heads are distinctive, sometime described as a goosefoot, or cat claw. As a result of centuries of cultivation, seeds heads can vary in colour and shape, including curved heads on this ornamental variety or straight heads.
Besides being a food crop, Eleusine coracana is also fermented and made into alcohol. I first encountered this plant when drinking tongba, or hot millet beer, while i traveled to dharan.

here's some picture of Eleusine coracana or simply the millets.