You go to the movie theater after eating dinner and smell the popcorn in the concession stand. although you are full, you start salivating and your stomach grumbles, so you buy a bag to eat during the show. this is an example of what signal for eating? \

This is an example of "appetite".

Appetite refers to the desire or want to eat food, some of the time because of hunger. Engaging nourishments can empower craving notwithstanding when hunger is missing, in spite of the fact that craving can be significantly lessened by satiety. Appetite exists in all higher living things, and serves to manage sufficient vitality admission to keep up metabolic requirements. 

The changes to agriculture in the united states since 1900 is that increased mechanization has increased food production, thereby increasing the need for labor. there has been an increased need for higher speeds of food production due the fact that the population in the united states continues to rise.