Can a human eat grass and digest it? Could it be possible to use it as food just like other plants such as wheat or beans?
To elaborate on A random zoologist's answer, the problem is that the human digestive system does not contain any cellulase enzymes. Cellulases are a class of enzymes that break down cellulose, the chief structural component of plants. You might be able to obtain a small amount of nutrition from grass or other cellulose-rich materials, but as the plant cell walls are made of cellulose, most of the plant material will be indigestible.
Wheat and other grasses are very significant sources for human nutrition. Grass seeds are digestible; seeds have to digest themselves to sprout, with very little biochemical machinery. Human civilization is founded on the ability to cultivate grass to eat.
It is due to pepsin that a cellulase supplement would not help to digest grass stems and leaves. Pepsin is the enzyme in the stomach that breaks peptide bonds and from its perspective, cellulase is just another peptide chain (pepsin is so effective, the stomach has to prevent itself from being digested while at the same time maintaining enough pepsin to digest food — because pepsin digests pepsin). Cellulose is a tough sugar polymer (as is chitin, which forms insect and fungus exoskeletons), so after all the cellulase was digested by pepsin only bacteria from the intestine could process the remaining cellulose. What the bacteria convert it to might make one ill.
Humans can derive significant nutrition from bamboo shoots, which have less robust cellulose than in mature plants, and bamboo is a grass. Pandas also cannot digest cellulose effectively, but that doesn't stop them from trying. They might eventually develop a cellulase gene by sheer willpower and tenacity.
@jarlemag: inhaling sugar water could quickly become a very bad idea. Encouraging microbes to colonize near the brain may result in them converting neurons into more microbes. The Naegleria amoebae can be very aggressive. In Pakistan and India it is somehow traditional to treat illness by putting water up one's nose; when it is not recognized how clean the water must be to do that safely, people die.
Well, humans can eat grass, but it will not be digested. Cause to digest grass our body needs different kind of enzymes, which lack in humans so they can't digest grass directly although humans can eat grass, but not recommended.
Similarly our body has different enzymes to digest wheat and beans etc. Also remember rice, wheat and beans are fruit, not plant. Although we can digest some of the plant.
Even in times of famine, when people are literally dying from hunger, they do not eat grass - it is not digestible. But they eat cooked artiplex, which commonly grows by roadsides.