In the human body, there are 2 factors which correlate in producing the well-known effects of hunger or being well-fed.
One is the glucose concentration receptor, which measures the amount of glucose in the blood. If this gets low, your body (and brain) "gets hungry" (shows the symptoms of hunger). This is the factor thatis primarily responsible for hunger.
The other kind is the mechanoreceptors in your stomach. They can sense the "fulness" of your stomach by sensing the tension of it. If the stomach is full, then the receptors convey the feeling of "being full" to the brain. This is the receptor that is primarily responsible for the feeling of "being full". (antagonist with the glucose concentration receptor)
In the brain, the signals fro the two kinds of receptors are getting summed, and the result is that you are either hungry or not.
Note that you can "deceive" your brain and body, for example by drinking huge amounts of water. Your stomach will be full with water, so the feeling of hunger will go away for a while.
Same is true for eating a spoon of sugar. The glucose receptors will measure a healthy amount of sugar despite your stomach being literally empty.